Dispelling Modeling Myths

[Harry P.]

So, what you're saying is that Squadron White putty can defy the laws of physics?

Apparently so. Must be magic.

[Danno]

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Fact: Squadron White Putty does not shrink or crack. Here's a photo of a hood I just primered. The tear drop scoop is made entirely from Squadron White Putty. This was started last night around 8:30 and primered just a few minutes ago. Myth busted!

1. Do you have precision micrometer measurements of several key dimensions upon completion of application, and correspnding measurements upon "drying" so that a meaningful conclusion might be reached as to how much shrinkage did or did not occur?

Perhaps Squadron White shrinks less than other formulations, or perhaps it shrinks so uniformly and so little that it is not detectable/noticeable by its true believers, but it has to shrink . . . it has no option. Squadron has no exemption from physics or chemistry.

2. Check back with us later . . . weeks, months, or years . . . we'll wait patiently.

[Art Anderson]

There's a few myths, or maybe "Half-truths" floating around on the forum about how people go about certain meathods or achieve certain results. Some of these may have some folks confused, or even scared to try a new technique. There are a few "myths" that I would like your help to get straightened out, and if you guys can think of more, please post them!

#1- Alclad, I've read on several instances where Alclad was hard to work with because of it being so delicate, but I've also read where it's just as tough as kit chrome... so which is it?

#2- Scribing open panels, BMF makes a tool just for this, yet guys insist on claiming to use the back-side of a #11 blade (which I can never get to follow the groove). How many of you actually use an Exacto blade to open panels?

#3- Fine line masking, again something that can be purchased, but when people ask the painter how they get such fine lines masked, they always come back with the old "stacking of the blades" response...

I don't want to start any arguements amongst the members, just would like to bring the truth about some things to light is all!

#1 I have known of Alclad for years now, never tried the stuff.

#2 I have, and have used, the BMF scribing tool, still the best on the market. However, I still use a modified razor saw blade whenever I need to scribe a long, straight door or other panel line.

#2, Fine (or narrow) masking tape: I've made my own for decades--a piece of thick tempered plate glass, a metal straightedge, and a #11 Xacto (no "E" in that name BTW), to cut my own fineline masking tape. It's worked for me through 40-some years of making multi-color paint jobs, starting with 20 years (1954-85) building models of Indianapolis cars--why should I change?

Really, I don't know of any "modeling myths", just that I along with countless other model car builders have techniques, materials, and tools that we've used for years that have served us pretty well, so why change (unless something we see as better comes along, or some "old friend" material or tool suddenly becomes unavailable. In other words, if it works, if it's still available, why change?

Art

[Art Anderson]

Apparently so. Must be magic.

All solvent-based paints and putties do shrink--they simply have to as the solvents evaporate (dry). Some shrink more than others of course. Even catalyzed putty will shrink ever so slightly as it "kicks" from soft to hard--that is a given there, as any polyester resin engineer will tell you. However, catalyzed putty shrinks by less than one-tenth of one percent, or 1/1000, and once cured (which happens in a small fraction of the time that it takes any solvent-based putty to dry completely) it doesn't shrink any more, nor does it crack when doing so.

With solvent-based putties (any and all of them) the shrinkage is related to the material thickness--the thicker the layer of raw putty laid down, the more (visibly) it will shrink. For that reason, I used very little of the stuff--in fact, most minor bodywork I do gets done with gap-filling CA glue, which once kicked with an accelerator, becomes hard, and doesn't shrink at all, along with filing and sanding to perfect feather-edging, and, in my experience, never shows through even a hot lacquer paint job.

Art

[Dr. Cranky]

I use a #11 blade to open panels. The trick is to go very very slowly at first and use a very light touch (light pressure on the blade). After a while you can press harder.

When masking, hit it with a little bit of clear first. That way the color coat will not bleed through. Any bleed through, under the tape, will be clear.

Another myth: You can paint and build a model car from sealed box to finished car in 24 hours. I am about to check out this myth. I'm signed up for my model club's 24 hour build fest.

.

LOL, you have to say abracadabra five times over the kit wearing foofy slippers in your pajamas while you open the kit in front of your neighbor's door.

[Draggon]

And as for the myth that Testor's Contour Putty shrinks forever, this chop was done around 1984. Very thin coats have minimal shrinkage, if any. I still use it today to fine-tune areas where primer alone isn't enough.

[Harry P.]

And as for the myth that Testor's Contour Putty shrinks forever...

Nobody ever said that one part putties shrink "forever."

They shrink until all of the solvent has evaporated. Once all the solvent is gone, they don't shrink any more. But the time it takes for all of the solvent to fully evaporate, and the shrinking to stop, varies greatly, depending on the brand of putty, the air temperature, how thick it was applied, the humidity in the air, etc.

That's where many people get into trouble... they think the putty has fully dried, and they go on and finish the model... only to see very visible signs of shrinking marring their paint job weeks or months later, especially along seam lines or joints or panel lines that were covered with the putty. Of course, by that point it's too late to fix the problem.

[Scale-Master]

4. Paint won't stick to polished plastic.

I figured that more people would comment on this one. To me, the conventional wisdom is that you need some "tooth" on the surface to be painted -- such as scuffing with 400 grit wet-or-dry for a coat of flat primer. On my next model, I'm going to try the polished plastic method and spray on many light coats of diluted gloss paint, skipping the primer.

How about posting some more myths?

It depends on the paint too. Mechanical vs. chemical bond. Mechanical bond paints need a texture or "tooth" to bite onto. Chemical bond paints "melt" or meld into the surface.

It also makes a difference if you are going to mask. For example, I have had great results with Tamiya TS paints on raw shiny (Tamiya) plastic, but they don't adhere well enough to mask without the tape pulling the paint off.

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

[Quick GMC]

I must point out, although it is nit-picky, that drying and curing are two completely different things. While a putty may be dry to the touch, it doesn't mean it is cured. Same thing with polyester resin, some epoxies, etc.

Putty shrinks as it cures. Maybe not much, but it does. Just because it feels dry, it doesn't mean is time to sand on it, correct? If it's cured, and ready to sand, it's done shrinking.

CA shrinks a bit too if you use the accelerator. It's an acrylic, but a bit different. It's an anaerobic sealer, it cures with the absence of air. It will sit in a cup for an hour without drying or curing, but stick two pieces together tightly and it cures instantly. I use it outside of the modelling industry a lot for surface repair, so I use the accelerator.

Putty shrinks, end of story. It shrinks as it cures. After it's cured (not just dry) it's done shrinking. If it cracks or pulls away after that, it could be from the climate, heat, UV light, etc.

Resin is not UV stable either, it will yellow, contract or turn brittle when exposed to sun light for a long time.

The shrinkage is minor. One way to gauge it is to fill a void or joint in a perfectly flat surface. Fill with putty and trowel it flush with the void or joint. Let it cure. You'll notice it is now concave, or below the surrounding surfaces.

I'm no chemist, but I work with adhesives for a living, I have learned a lot over the years.

Edited May 16, 2013 by Quick GMC

[Harry P.]

The shrinkage is minor. One way to gauge it is to fill a void or joint in a perfectly flat surface. Fill with putty and trowel it flush with the void or joint. Let it cure. You'll notice it is now concave, or below the surrounding surfaces.

Exactly.

One-part putties shrink as they dry. Period.

[mistermodel]

would using a food dehydrator fully cure a putty so you could carry on with your body work?

[Porscheman]

4. Paint won't stick to polished plastic.

I figured that more people would comment on this one. To me, the conventional wisdom is that you need some "tooth" on the surface to be painted -- such as scuffing with 400 grit wet-or-dry for a coat of flat primer. On my next model, I'm going to try the polished plastic method and spray on many light coats of diluted gloss paint, skipping the primer.

How about posting some more myths?

I use Novus #2 for polishing before painting my final coat. Never had a problem with paint sticking. Novus can be found at good LHS and glass stores that sell plexiglas.

[Harry P.]

would using a food dehydrator fully cure a putty so you could carry on with your body work?

A dehydrator would speed the process of the solvent evaporation, yes. Exactly the same way as it speeds the process of paint drying (solvent evaporation).

But there's really no way to be sure that the putty has fully cured. That's why 2-part putties are a much better choice, in my opinion. Once they've cured, they've cured. Period. The chemical reaction has taken place and there will be no shrinking. With 2-part putties you don't have the long wait or the uncertainty as to whether or not the putty has truly cured completely.

[Quick GMC]

Accelerating the curing process, which is a chemical process, may result in more shrinkage, and/or a more brittle surface. They are designed to cure a certain way, start messing with that and you may not like the results, or it may be fine.

Acrylic and polyester resins use a hardener. There is a ratio you are supposed to follow, but some people add more. It usually around 2-5% hardener ratio to resin. If you add more, it will cure much faster, but it will shrink more and become brittle.

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.

[Scale-Master]

I use Novus #2 for polishing before painting my final coat. Never had a problem with paint sticking. Novus can be found at good LHS and glass stores that sell plexiglas.

Just to be clear... Is that paint to paint for the last coat? Paint will adhere much better to polished paint than polished plastic.

[Porscheman]

Just to be clear... Is that paint to paint for the last coat? Paint will adhere much better to polished paint than polished plastic.

That is paint, polish and then paint. I always use 2400 cloth on bare plastic prior to 1st coat of paint. I never tried Novus on bare plastic before painting though.

[Scale-Master]

Thanks. That makes good sense.

Novus does work great on plastic, but I wouldn't use it for paint prep on raw plastic either...

[Harry P.]

Accelerating the curing process, which is a chemical process, may result in more shrinkage...

The drying process for 1-part putties is not a chemical process, it's a physical process (evaporation). Same as paint. The dry heat of a dehydrator simply speeds up the process of evaporation.

Two-part putties cure via a chemical process.

[Quick GMC]

The drying process for 1-part putties is not a chemical process, it's a physical process (evaporation). Same as paint. The dry heat of a dehydrator simply speeds up the process of evaporation.

Two-part putties cure via a chemical process.

It still off-gasses. It's curing, different than drying. I may be wrong, I'm not really sure on this one to be honest

Edited May 16, 2013 by Quick GMC

[Scale-Master]

Heat can speed up the gel or kicking of the two part fillers. But since it is a chemical reaction, it would be safer to just let it run its course. As discussed, dry and cure times are not the same. Heat can make it feel dry sooner, but full cure is when it is most stable.

[JollySipper]

Guys, I thought that using a polishing compound before a coat of paint was a no-no... Seems like it would cause a reaction to the finish? Or is that another myth?

[Harry P.]

It still off-gasses. It's curing, different than drying. I may be wrong, I'm not really sure on this one to be honest

You can call it curing but it's actually drying. Technically drying via evaporation (the solvents, that is)... and evaporation is a physical process, as opposed to the chemical reaction that takes place in 2-part putties. Heat and dryness speed evaporation, so putting a 1-part putty in a dehydrator would definitely speed up the drying time.

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).

[Longbox55]

Guys, I thought that using a polishing compound before a coat of paint was a no-no... Seems like it would cause a reaction to the finish? Or is that another myth?

Depends on the compound used. Some are made just for that, and are paintable.

[Quick GMC]

You can call it curing but it's actually drying. Technically drying via evaporation (the solvents, that is)... and evaporation is a physical process, as opposed to the chemical reaction that takes place in 2-part putties. Heat and dryness speed evaporation, so putting a 1-part putty in a dehydrator would definitely speed up the drying time.

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).

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.

[Quick GMC]

When using two part adhesives, you can accelerate the curing time by mixing it faster or longer, which will generate more heat and accelerate the curing. Any resin, that uses a hardener, can be accelerated with a heat gun (hair dryer is too cool) or an open flame. We use a weed burner propane torch in cold weather.

resins are extremely reactive to temperature. The difference in working time can be anywhere from 5 minutes to over an hour in the summer/winter. depending on the chemical. Most of the ones used for hobbies or general use are room temperature resins and are designed to be used in that mid range.

Edited May 16, 2013 by Quick GMC