"Any chemists that know why an acrylic primer is so stubborn?" Topic

I had a kind of a strange experience while stripping some minis. recently. I stripped some GW plastics with Super Clean. They had a coat of Army Painter white spray primer and a coat of brush on white primer over that.

They were in the Super Clean for about three days. Usually that is easily more than enough time to completely strip away whatever is sticking to a plastic mini. But after the third day only the second layer of brush on primer had actually lifted. The spray primer had stubbornly remained in place and in fact was still resistant to the scratch test even.

Earlier this year I used Super Clean to strip more GW plastics. They had a bad primer coat of Rustoleum X2 Paint + Primer. That was a green color. I had those in Super Clean for about three days and it lifted off the spray coating so well I didn't even have to scrub them, just rinsing them and the models were back to bare plastic.

More recently I stripped some movement trays that I had spray primed literally about ten years ago with some brand of brown spray paint. Even a ten-year old spray coat lifted off in a matter of a day, totally gone.

Can anyone tell me why of all the things I stripped with Super Clean, that a white acrylic spray primer would be the most resistant to stripping? I would have thought that the movement trays which had spray paint that had cured for a decade would have been the hardest to strip! I also find it bizarre that it could resist Super Clean so well after having been on the model for less than a day and didn't even have time to fully cure.

Maybe Army Painter sprays are just really good products that adhere very quickly! I do know they dry at the speed of light, some of the fastest drying spray primers I've ever seen.

…I'm thinking maybe because Super Clean is a degreaser, and it's more designed to break up enamel-based sprays and enamel-based paints? So an acrylic primer or paint is more resistant to being stripped?

The differences are difficult to address, as the exact mixture used in any particular brand (or even a batch within a brand) is not made public.

The drying time is really not an issue. That is simply the time required for the bulk of the solvent to leave the body of the paint. The critical issue is the polymerization of the binder. For a very loosely "netted" polymer the entrapped solvent will weaken the resulting sheet, so greater "drying" will result in additional strength and resistance to breakdown. For a well "netted" polymer, the maximum strength can be reached quite quickly. And the paint will still contain a large percent of solvent.

Binder is the key ingredient and this can vary greatly from 10% to perhaps 33% of the bulk of the paint. Primer, and spray primer in particular, will typically have a great percent binder.
Also, spray clear over coatings will also tend to have the highest percent binder.
The binder itself is the monomer that will polymerized (gel or cure) to from the structural part of the paint.
Typically ethenyl ethanoate (vinyl acetate) and a propenoate (acrylic) ester are the main parts of the binder with
co-monomers with ethenyl ethanoate are ethyl propenoate, butyl propenoates, or a co-polymer of butyl propenoate and methyl 2-methylpropenoate. These co-polymer typically are strength by adding cross linkages in the net of the polymer. But are a very small part of most paints, perhaps 0.1% to 5%. The pigment and the additives to create the matte finish of primers are typically 25% to 37% of the bulk. But variations can be very great.

I suspect your resistant primer was a combination of higher than intended binder and perhaps a higher percent of co-polymer as well. This would form a particularly tight net of polymer which would be more resistant to the attack of your stripper. Super-Clean being water based, it might have had a particularly difficult time with the tight polymer net.

You results with another can of the same brand of primer may be very different.

Wow, thank you that's a very impressive answer. Much appreciated!

Thanks for taking the time to describe the dynamics of it in so much detail, really great information.

:)

Dyehard,

One follow up question if you don't mind…

There was still pretty much a solid coat of spray primer covering significant parts of the models after I finally concluded the stripping. I rinsed them thoroughly and put them out to dry.

After they had dried I put a fresh coat of spray primer on the models again.

My question is, will the new coat of spray primer adhere properly over the original coat now that it's been subjected to a stripper, or do you think there will be paint adhesion issues? As I said it appears that Army Painter's white primer any way achieves that bond very, very quickly.

I just tested the surfaces with a finger nail and the new coat of primer isn't even budging, so it all looks pretty good.

When a lower layer of paint is well adhered to a surface, it will not be a problem for the next coat of paint. If you scratched up the lower primers the surface smoothness will be affected. Just make sure the surface if dry and free of grease.

I was thinking it was odd that you were stripping primer. Although I know sometimes a person puts down too thick of coat of paint and thus loose some detail.

Stripping paint for a metal figure gives you more options. But if you really need to strip plastic, some of the options out there include: Dawn Power Dissolver, Simple Green (although I have heard they changed formula so not as good), and Pine Sol concentrate. Pine Sol is my go to, the smell is a bit much. I normally soak for 24 hours or more. Pine Sol does seem to soften plastic at first (this might be my paranoia) but let dry for a week and all seems fine in the end.

Thanks again!

I use Super Clean because of a recommendation I got from several different modeling forums. As I said it worked fabulously on other model plastics.

Yeah it was odd that I was stripping primer, probably an OCD thing on my part. I had sprayed the initial coat of white. Then after only about 20 minutes I applied the coat of brush-on primer. (I'm doing some undead skeletons and I generally do both a spray coat and then a single coat of brush-on primer so that I get a good, solid white finish all over to apply washes over. I also do that with models that are all white like Star Wars Stormtroopers for example…)

It was Army Painter white spray. It was probably fine to apply the brush-on primer right after the spray primer. Army Painter dries lightning fast. According to their web site you can paint over their spray primer pretty much as soon as its dry, like 15-20 minutes. So as far as they are concerned the whole waiting for a day thing seems to not matter for their products. But then I was thinking about general rule of the "24-hour"waiting period of letting a coat of spray primer cure for about a day before doing anything over it.

So in my OCD doubt I decided to strip it and do it again.

I'm wondering if in most cases you can paint over a spray primer sooner than a lot of modelers wait for. I guess that 24-hour wait has become a modeling standard. I adhere to that nearly 100% of the time. But this time I was just in a groove where I did it before I realized I hadn't waited.

I am curious if Acetone/Nail Polish Remover, would lift that tough primer better? Not a chemist, just wondering, as Acetone is often touted as a great stripper for Mage Knight figures' factory paints. Cheers!

@Sgt Slag,

Quite possible, if it's been proven to lift paint off of Mage Knight stuff! That kind of factory paint would be very well adhered to the products so it's definitely worth a try in the future I think. Thanks for the suggestion!

Sgt Slag:

Note that acetone will attack many plastics, so not good for the original poster's case "some GW plastics".

Acetone is a very good mid polarity solvent. Its low viscosity means it can work its way into, under, and in-between things. But it is also very volatile and flammable.

The difficulty with removing paint is that once it is on, it is a sheet of polymer, and this polymer is a function of the bonding media of the particular paint. Acetone does soften acrylic, but not as fast as it does polystyrene (typical plastic in models and plastic figures).

If you are on metal you can boil the SuperClean/Simple Green/PineSol to aid in getting the paint off. But with Acetone being so low boiling temp and flammable it is much more difficult to work with.

Never thought of boiling Simple Green/Pine Sol/Super Clean… Or Acetone. I knew Acetone was volatile, but I had no idea it has a low boiling temperature. I know so little on chemistry, that I would not "experiment" with unknown chemicals, because I might hurt myself, and others… Thanks for the info. Cheers!

You can find out safety info on a pure chemical quite readily.

Type the chemicals name and the letters "SDS" into a search engine. It will give you the "Safety Data Sheet"
"Acetone SDS"
link

It boils at 56 °C (133 °F).

Heating a solvent will tend it make it more "aggressive". But avoid this for flammable solvents unless you have been trained. But water based things like PineSol can be heated without too much hazard other than getting burned.