Dispelling Modeling Myths

[Harry P.]

It's still a chemical process though. It does dry, because it's not wet any more, but a chemical is evaporating from it, changing the chemical properties. That's how it gets hard, instead of just drying into a brittle dust.

Thin set for setting tile is another example. You add water, then the water evaporates, yet it's not just drying, it's curing. It off-gasses.

Ok, I see your point, but then anything that undergoes "drying" is technically undergoing a chemical change, isn't it?

But scientific hair-splitting aside, my point is that 1-part putties and 2-part putties "set" (or "dry" or "cure" or whatever word you want to use) via completely different processes, and the process by which 1-part putties set (evaporation) results in shrinking, while the chemical reaction that happens as 2-part putties set does not result in any appreciable shrinking.

Two completely different curing processes.

[Quick GMC]

Yes, you're correct.

You can't lump two part putties together though, because they aren't all the same. 2 part resins shrink because of the binders that are used, and when the curing starts, they pull together much more, like hugging each other, so it contracts as it cures. With hybrid epoxy resins, they don't contain styrene. They can shrink a small amount, but not much, they don't squeeze together like the polyester versions. True 2-part epoxies and urethanes won't shrink

Not that any of this matters, just fill the hole and sand it down.

[Ace-Garageguy]

2-part putties cure, they don't "dry." They cure via a chemical reaction; this reaction takes place no matter what the ambient temperature or humidity may be (although the speed at which that reaction takes place can be slightly affected by temperature). But even in the cold, that chemical reaction will occur (assuming the correct ratio of resin to hardener).

Epoxies are different. They are almost always 1:1 or 2:1 ratios. If you veer off from this, it just won't get hard. That's why you get more consistent and accurate results.

Just to be perfectly clear and true to my pedantic nature, curing of the 2-part materials we use is referred to as an 'exothermic' reaction, meaning that as the chemical bonds are formed, heat is given off. In many cases, a certain minimum threshold temperature is required to initiate the reaction...usually about 60deg F. Cure times can be WILDLY effected by ambient temperature, as the material exotherms and creates more and more heat, accelerating the reaction...a dangerous feedback loop. I've seen over-catalyzed material start smoking and burst into flame on hot days.

Polyester based products (2-part glazing putty, bondo, etc.) typically contain styrene-monomers as thinners and crosslinking-agents, and this is the stink you get working with non-epoxy fiberglass. These solvents are reactive and are mostly incorporated into the hardened material, but some DO evaporate out of catalyzed polyesters, like bondo, and cause minor shrinking as they leave. BUT, because they occur at a much lower concentration than the solvents in 1-part putty, shrinkage is vastly reduced.

Epoxies come in a WIDE array of mix ratios, and the higher strength the particular epoxy is, the more critical it is to mix it by weight, correctly. Structural aircraft goo must be maintained to + or - 1/2 of 1%. The two most commonly used in my current work mix at 5:1 and 1.4:1 ratios.

MOST thermoset plastics (those that cure by exothermic reaction) will benefit from POST-CURING at elevated temperatures AFTER they have hardened to touch at room temperature. MGS 285 for instance, a common aviation epoxy, needs a room-temp-cure of 24 hours, and a post cure at 140deg F. for 15 hours to achieve full strength.

Edited May 16, 2013 by Ace-Garageguy

[Tom Geiger]

What I have to add to the subject:

1. Alclad - This is also available in a spray can. Many folks don't seem to know this. I think it works just fine and I don't think the surface rubs off unless you are a polish fiend. I have also sprayed a little into a paper cup and used it to touch up kit chrome and add bits like key holes and door handles.

2. Scribing open panels - Oh, kill me but I use my small battery powered eXacto brand drill to do this. I got this drill over 20 years ago using a gift certificate my inlaws gave me to a hobby shop. It's my favorite tool and I use it every day. I haven't seen anything like it for sale for many years. I use a small drill bit and perforate it in the panel line. Once I go all the way around, I will scribe it from the back (as in connect the dots) and carefully push the panel until it comes out. Once I get a little corner going, it pretty much tears out like a postage stamp. Once the panel is out, I just sand all the edges flat for a good fit once painted.

3. Fine Line Masking - I have nothing to add further than the others have already covered nicely.

4. Build a model in 24 hours - YES! I did the 24 Hour build this past February with Gary Kulchock and gang on their Facebook page. I did manage to build up my Miss Deal Funny Car within the time allowance. Started at noon on Sat and finished (gave up!) at 5am on Sunday morning.

Here it was at the 24 hour deadline. Later on I added the headers (people warned me about these), the scoop (kit one had a big sink on it, the windshield and push bar. I also added some rust weathering later on.

Edited May 16, 2013 by Tom Geiger

[Harry P.]

But the bottom line, as it applies to our usage, is that once the chemical reaction in 2-part putties has happened, and the resin and hardener have combined to form a new chemical compound, that compound is stable and there is no further shrinking, whereas one-part putties can continue to shrink for a long time after application

Correct?

[Quick GMC]

correct, it is the curing and/or drying that creates the shrinkage. Theoretically, once it's dry enough to touch without it being tacky or no longer malleable, it is done shrinking. This can vary with certain products.

[Ace-Garageguy]

But the bottom line, as it applies to our usage, is that once the chemical reaction in 2-part putties has happened, and the resin and hardener have combined to form a new chemical compound, that compound is stable and there is no further shrinking, whereas one-part putties can continue to shrink for a long time after application

Correct?

That's pretty much it. The styrene monomers in 2-part putty are 'reactive diluents' and are mostly 'used up' during the curing reaction, and what's left is such a small percentage of the product that shrinkage is virtually nil on models.

One BIG problem with 1-part putties is that they skin or surface dry adjacent to the air first, and this inhibits further evaporation of the solvents. This is the reason why applying in THIN layers is better, if you insist on using the stuff, and to delay primering as long as possible to allow for continuing evaporation.

IF the 1-part stuff is painted before being COMPLETELY dry, paint itself slows the evaporation of the solvents in the filler, but it's slightly porous and STILL allows very SLOW evaporation to continue. I DO have models that have shown surface flaws due to shrinkage after 6 months or more, when the final finish was perfect when the model was completed. Sometimes you have to look under magnification to see it, or sometimes it will only be really apparent in certain lighting conditions.

Edited May 16, 2013 by Ace-Garageguy

[Harry P.]

That's pretty much it. The styrene monomers in 2-part putty are such a small percentage of the product that shrinkage is virtually nil on models.

One BIG problem with 1-part putties is that they skin or surface dry adjacent to the air first, and this inhibits further evaporation of the solvents... IF the 1-part stuff is painted before being COMPLETELY dry, paint itself slows the evaporation of the solvents in the filler, but it's slightly porous and STILL allows very SLOW evaporation to continue. I DO have models that have shown surface flaws due to shrinkage after 6 months or more, when the final finish was perfect when the model was completed. Sometimes you have to look under magnification to see it, or sometimes it will only be really apparent in certain lighting conditions.

And that is the reason that 2-part putties are the better choice for us model builders–to avoid the "surprises" down the road that can happen with 1-part putties.

[MAGNUM4342]

#1. As the others have said, Alclad is tougher than kit chrome...IF YOU APPLY IT RIGHT. Those of you who have it rubbing off aren't applying it right.

#2. I use an Exacto blade or a dental pick. Either works great for opening panel lines. I definitely prefer the dental pick. It rarely slips out of line.

#3. Sometimes masking tape, sometimes frosted cellophane tape and sometimes bare metal foil.

The main thing to remember is not to stir the pot by alluding to- some folks are lying or talking out their arse when asking such a question. Remember, everyone is different and results WILL vary person to person. What works for me might not work for jollysipper. When one makes statements like "I just want to bring the truth to light..." it flies in the face of "I don't want to start any arguments...".

Focus on trying a new technique as described by a member. If it doesn't work for you after a few tries, go to a different method. It's as simple as that. If you are scared to try a new technique you are most likely trying it on the wrong model. Try it first on a kit you don't care about, or even better, a plain sheet of Plastruct.

[zenrat]

...What about the myth of red and yellow plastic bleeding through paints...?

That's no myth. Well, with red it's true - I haven't built a bright yellow kit since I was a kid.

If you don't use some method of sealing the red in then the colour will bleed through light coloured paint.

[Harry P.]

That's no myth. Well, with red it's true - I haven't built a bright yellow kit since I was a kid.

If you don't use some method of sealing the red in then the colour will bleed through light coloured paint.

Is it really the plastic "bleeding through?" Or more a case of a light color paint needing more coats to fully cover a brightly-colored plastic?

[sjordan2]

Is it really the plastic "bleeding through?" Or more a case of a light color paint needing more coats to fully cover a brightly-colored plastic?

Most of what I've learned here and on other forums tells me that, without a proper original barrier like Future, the red will "bleed" through, causing white paint jobs to turn pink in a short amount of time. I've seen this over and over on many forums. And the fact is, many of my kits that are molded in red feel a tiny bit oily to the touch, unlike other molded colors. That may be a case of prejudiced perception, but that's what I've noticed. I haven't seen similar concerns about yellow.

Time for Donn Yost to pipe in. And Ace, our resident chemist, probably knows all about this.

Edited May 18, 2013 by sjordan2

[Tom Geiger]

Most of what I've learned here and on other forums tells me that, without a proper original barrier like Future, the red will "bleed" through, causing white paint jobs to turn pink in a short amount of time.

Yup. I have a pink Cord downstairs that I got in a lot of 1930s classic models.

[Art Anderson]

Is it really the plastic "bleeding through?" Or more a case of a light color paint needing more coats to fully cover a brightly-colored plastic?

I seem to recall that years ago, colored plastic never "bled" through white paint (I painted the original-issue -- molded in red plastic -- AMT Double Dragster Fiat Coupe with the early AMT lacquer in white, with no bleed-through whatsoever. It could well be that early on, styrene was colored using pigments as opposed to dyes which I suspect is what is used today. Other than that, I have no explanation, not even any real idea.

Some paints will bleed through white as well, most notably white (had that experience with AMT white lacquer over their red lacquer on an MPC "Rislone Eagle" Indy car perhaps 40 years ago or so--painted the red first, masked off, and painted the white portion of the nose and tub. The white turned pink--guess what I never did with white paint from then on!

Art

[Danno]

Dispelling another modeling myth: There's no "E" in the product brand name "X-acto."

[Harry P.]

Dispelling another modeling myth: There's no "E" in the product brand name "X-acto."

And no "I" in team?

[mistermodel]

And no "I" in team?

True,but there is ME lol

[Pete L.]

What I have to add to the subject:

1. Alclad - This is also available in a spray can. Many folks don't seem to know this. I think it works just fine and I don't think the surface rubs off unless you are a polish fiend. I have also sprayed a little into a paper cup and used it to touch up kit chrome and add bits like key holes and door handles.

2. Scribing open panels - Oh, kill me but I use my small battery powered eXacto brand drill to do this. I got this drill over 20 years ago using a gift certificate my inlaws gave me to a hobby shop. It's my favorite tool and I use it every day. I haven't seen anything like it for sale for many years. I use a small drill bit and perforate it in the panel line. Once I go all the way around, I will scribe it from the back (as in connect the dots) and carefully push the panel until it comes out. Once I get a little corner going, it pretty much tears out like a postage stamp. Once the panel is out, I just sand all the edges flat for a good fit once painted.

3. Fine Line Masking - I have nothing to add further than the others have already covered nicely.

4. Build a model in 24 hours - YES! I did the 24 Hour build this past February with Gary Kulchock and gang on their Facebook page. I did manage to build up my Miss Deal Funny Car within the time allowance. Started at noon on Sat and finished (gave up!) at 5am on Sunday morning.

Here it was at the 24 hour deadline. Later on I added the headers (people warned me about these), the scoop (kit one had a big sink on it, the windshield and push bar. I also added some rust weathering later on.

Tom,

Sounds like the Miss Deal kit was a "plumber's nightmare" !

Pete L.

Edited May 20, 2013 by Pete L.

[Tom Geiger]

Tom,

Sounds like the Miss Deal kit was a "plumber's nightmare" !

Pete L.

Hi Pete!

Miss Deal was an interesting build. For the 24 Hour build, I thought I'd find a kit that I always wanted to build and had multiples of, so I wouldn't fear messing one up. I also needed a kit simple enough to finish in the allowed time. Per the rules, it had to be a full detail kit, so no curbside snappers!

The kit itself was typical Revell of the era. Lots of mold lines and ejector pin marks. Assemblies were unnecessarily complicated, like the engine being many pieces that went together poorly. The directions also were sketchy, leaving some of the build to interpretation. No wonder kids gave up on these in the 1960s! I'm an adult with an engineering background and I wound up doing some steps twice. If I did another one of these, I wouldn't use the kit engine.

The 24 Hour build did teach me to build faster when necessary. And instead of throw away results, I actually like the car! Finished car below.

Edited May 21, 2013 by Tom Geiger

[AzTom]

Here is what I use. Bondo. A two part putty made for car body repair. It now comes in these tubes that are very handy for model car building. Available at most auto parts stores. The only problem is, if you mix in too much hardener (stuff on the right) it will set up in about 3 minutes.

Jim, The Bondo brand Spot & Glaze putty you show here does not use a hardner. It will set in minutes right out of the tube if used as intended, thin coats.

[AzTom]

#1 I have had good results with Alclad although I have not used on the cars I play with.

#2 I also use the #11 blade backwards, but snap about an 1/16in of the tip off first. Easier to control and cuts better.

#3 I like the Line-O-Lot tapes, comes in sizes from .10 to 1/2 in wide.

[Tom Geiger]

#2 I also use the #11 blade backwards, but snap about an 1/16in of the tip off first. Easier to control and cuts better.

I learned that one by accident! You are correct!

[Longbox55]

Jim, The Bondo brand Spot & Glaze putty you show here does not use a hardner. It will set in minutes right out of the tube if used as intended, thin coats.

I use the same putty, it does require a hardener, as it is a 2 part putty. There is a single part putty made by Bondo, that is in a similar tube.

.

[AzTom]

Jim, The Bondo brand Spot & Glaze putty you show here does not use a hardner. It will set in minutes right out of the tube if used as intended, thin coats.

I use the same putty, it does require a hardener, as it is a 2 part putty. There is a single part putty made by Bondo, that is in a similar tube.

.

You do learn something here every day. I have been using Spot & Glaze putty on models and real cars for 40 years and never saw a two part type.

The tube I have looks like that tube, but not sure about the "Professional" name. That said, it's probably 10+ years old. Mine is red and dries in minutes. I like it because you don't have to mix it and it dries quick.

[zenrat]

Is it really the plastic "bleeding through?" Or more a case of a light color paint needing more coats to fully cover a brightly-colored plastic?

It's the plastic (or some chemical contained therein) bleeding through.

I know this because I have sealed red plastic with tamiya primer, put white overcoat on top which remained white and then hit it with too heavy a coat of clear that caused the whole thing to turn pink.

Point being that no red showed through the white until I put down the clear.