Stripping pre-painted kits

[khier]

Dear all,

 

I have a couple of prepainted kits that I would like to repaint. Along time ago I tried brake fluid with one of them but did not help. Any other suggestions?

Regards

 

Walid

[Mark]

91% rubbing alcohol has worked on those every time I have tried it, within minutes (sometimes faster).

[khier]

1 hour ago, Mark said:

91% rubbing alcohol has worked on those every time I have tried it, within minutes (sometimes faster).

Did it work with AMT/Revell prepainted kits?

[Mark]

I don't think I ever had a prepainted Revell kit.  But, I have used the 91% alcohol on several AMT prepaints with excellent results.  Paint often falls off in seconds, and a good scrub with a soft toothbrush gets all of it.  

It has to be 91% rubbing alcohol.  Lower percentage concentrations won't do anything.

As for brake fluid, I haven't used it in many years so I can't comment on it as a paint removal agent.

[NOBLNG]

Brake fluid will take just about anything off. I just stripped some parts that had an unknown glossy paint on them. Super Clean didn’t touch it. IPA softened it so I could scrape it off, and brake fluid took it right off. I use it as a last resort because it is nasty to dispose of. I have never tried oven cleaner.

[khier]

13 minutes ago, NOBLNG said:

Brake fluid will take just about anything off. I just stripped some parts that had an unknown glossy paint on them. Super Clean didn’t touch it. IPA softened it so I could scrape it off, and brake fluid took it right off. I use it as a last resort because it is nasty to dispose of. I have never tried oven cleaner.

Oven cleaner never worked with me as paint remover. IPA worked sometimes, sometimes not. Brake fluid has been always my first choice. However, it did not affect the paint at all when I tried to strip Revell's BMW Z3 prepainted kit several years ago.

[Mark]

I'm not wild about brake fluid for stripping paint.  Besides the disposal issues, both the brake fluid and the plastic going into it are essentially based on petroleum in some way, and the brake fluid will either get into the plastic or take something out of it.  My experience with it has often left the plastic more brittle, even with minimal exposure.

[Bainford]

20 hours ago, Mark said:

I'm not wild about brake fluid for stripping paint.  Besides the disposal issues, both the brake fluid and the plastic going into it are essentially based on petroleum in some way, and the brake fluid will either get into the plastic or take something out of it.  My experience with it has often left the plastic more brittle, even with minimal exposure.

Actually, brake fluid is specifically non-petroleum. One of the design requirements for brake fluid is that it is non-injurious to plastics and plasticized rubber-like materials that make up the seals in the brake system (though this may not necessarily include poly-styrene). Depending on the grade, brake fluid is either an alcohol/glycerin blend, or alcohol/borate ester blend, with various performance additives. The types and quantities of additives used will vary from one brand to the next, and I suspect it is one of these additives that causes the brittleness some people complain of (some brands/grades being worse than others). The specific plastic formulation of the kit in question may also be a factor.

Given these variables, using brake fluid may be a hit-or-miss proposition. Personally, I've not experienced brittleness. Back in the 80s, concerned about this very thing, I put some brake fluid in a bottle and dropped in a wheel as a test. A few months later I checked it, and it seemed to have suffered no ill effects. So I left it for a couple years and checked it again, and its integrity remained intact. The grade and brand of brake fluid used is long forgotten. I think it's time I performed the experiment again, with something closer to a scientific method, as the 'brittle' comments have me curious.

So far, I've had no qualms about using brake fluid to strip paint. My go-to is Castrol Super Clean or 91-99% isopropyl alcohol, depending on the paint type. If the paint refuses to move, I dunk it in the brake fluid for a couple days to a couple weeks, depending on how stubborn the paint is. I make a trip once a year to the local haz-mat disposal centre to get rid of used motor oil, coolant, old gas, etc, so disposal is not a problem. My only concern is to ensure the body is scrubbed clean very well with soap and warm water once removed from the brake fluid tub.

[peteski]

On 11/24/2022 at 11:47 AM, khier said:

Oven cleaner never worked with me as paint remover. IPA worked sometimes, sometimes not. Brake fluid has been always my first choice. However, it did not affect the paint at all when I tried to strip Revell's BMW Z3 prepainted kit several years ago.

You are in Germany, so the oven cleaner where you are might not be the same as what is available in USA.  Actually, even in USA, there are oven cleaners which work, and others which do not work.  The active ingredient that strips the paint is Sodium Hydroxide (also called Lye in English).  Check if your oven cleaner contains Lye.  If it does, it might not be as concentrated as in the American version.

As for IPA, the stronger concentration, the better.  70% is useless, 91% works better, but 99% is the best.  In USA, I can buy 99% Isopropanol (pure IPA) in quart size cans at a local hardware store.  It is usually displayed in the paint thinners isle.  If the store does not stock it, they can special order it for me.  Of course the price has  really gone up since COVID.

On 11/25/2022 at 9:09 AM, Bainford said:

Depending on the grade, brake fluid is either an alcohol/glycerin blend, or alcohol/borate ester blend, with various performance additives. The types and quantities of additives used will vary from one brand to the next, and I suspect it is one of these additives that causes the brittleness some people complain of (some brands/grades being worse than others). The specific plastic formulation of the kit in question may also be a factor.

Glycerin?  Borate Esters?  Did you maybe mean "glycol"?
DOT3 brake fluid we use to strip paints consists of mainly Diethylene Glycol and Butoxytriglycol.  See http://championbrands.com/MSDS/DOT3BrakeMSDS.pdf  .  As you mentioned it can affect certain formulations of polystyrene or ABS models cars are made of.  Actually the "official" plastic safe model paint strippers like PollyScale/Testors ELO uses similar family of chemicals (Isobutyl Alcohol, Diethylene Glycol Monobutyl Ether). But even ELO has affected some polystyrene I stripped in it.  Each stripping job is unique. and one has to be careful using "plastic safe" paint strippers.

Edited November 30, 2022 by peteski

[Ace-Garageguy]

On 11/25/2022 at 9:09 AM, Bainford said:

...The specific plastic formulation of the kit in question may also be a factor.

..I think it's time I performed the experiment again, with something closer to a scientific method, as the 'brittle' comments have me curious.

I stripped a vintage AMT Indy car in DOT 3, pulled it out after a day, and it literally crumbled.

I've also stripped an automotive-grade catalyzed urethane clear from an old Revell Chevy panel delivery with no adverse effects whatsoever...and the DOT 3 was the only thing that would even slightly soften the paint...but I still had to carefully scrape it with an X-Acto chisel-tip blade.

In my experience, high-proof iso usually strips factory finishes on cars and model railroad equipment...and some lacquers.

Oven cleaner takes off old enamels, usually, even stuff that's been on a model for 30+ years, but often takes several applications on the really old finishes.

I'll only use brake fluid as a last resort.

Edited November 29, 2022 by Ace-Garageguy