Paint problems on plastic ...can paint-stripper prevent adherence?

[nickelplate759]

I've been having intermittent paint adherence problems lately - like for a couple of years.  I'm using Tru-Color paint.  The problem seems independent of color.

The paint lays down and dries OK and looks just fine, but when I mask for a second color coat the paint sticks to the tape (Tamiya) and lifts in big patches.  This can happen even before I burnish the tape down, such as when  lightly put the tape in place and then lift it to reposition.
Here's the latest example (happens to be an old Rowa coach, but there's no pattern as to the model manufacturer that I can see):


The models have been cleaned prior to painting, with Dawn Ultra or a similar dishwashing liquid.

The problem usually (maybe always, w/one exception, see below)  happens when I'm painting on bare plastic that I have stripped of factory paint.  I'm not having problems if I just paint over the existing paint.   The stripper in question has been Scalecoat Wash-Away.


I'm wondering if Wash-Away leaves some kind of residue in the plastic that might cause this, or is the Wash-Away a red herring?
I did have this problem on one model I did not strip - it was a resin casting (from @skytop35 ) , and my thought at the time was that maybe I didn't clean it well enough, but now I'm not so sure.

For the existing messed-up models, I will strip and repaint, but don't want to repeat the failure.  I wondering if using a different kind of paint as a primer (maybe Tamiya or Mister Surfacer, from a rattle-can) might prevent the problem.

So - does anyone have any knowledge or similar experiences?   If the problem is caused bye the Wash-Away, would ELO cause a similar issue (important, since Wash-Away is no longer available anyway), or is it different enough to be OK?   Or is it just that the paint stripper needs something other than detergent to neutralize it?

[Bendtracker1]

When I was painting my U33B's last summer, I was having the same issue.
The shells started out as undecs, I cleaned them and used gloves.  I used Tru-Color Rock Island Maroon and Rock Island Red for the base coat.           
I left the first coat to dry overnight.  Then I used the Tamiya tape to mask for the Rock Island Yellow on the ends.

When I peeled tape away (at a slow, sharp angle) some of the Maroon and Red paint peeled or flaked away with the tape, though not as bad as you show.

When I painted my GP35 later, I decided to use some of Tru-Color's Light primer.
Repeated the masking as before and not one flake of paint came up?
Not sure if the primer helped or I just got lucky.
But I plan to continue to use their primer for upcoming projects.

[learmoia]

I have not painted with Tru color paint, but my guess is their Railroad colors like a base coat of primer to aid in adhesion to bare plastic.

If you strip, clean, prime, paint and still have pull away after a mask, then I'd guess the problem is with one of the products in the strip cleaning process.

Also.. any remaining 'old' paint may retain stripper that could react to the new coat.

Another thing you could test.. is take a junk stripped body and spray half with primer.. then paint the body with 2-3 brands of paint over the half primer half plastic body..
So you should have
w/ Primer: Brand 1  Brand 2  Brand 3
no Primer: Brand 1  Brand 2  Brand 3

Then layout a piece of masking tape across the whole thing burnish it then pull it up.. and see what sticks and what doesn't.

~Ian

 

[bbussey]

Always prime when using Tru Color if you’re going to mask. I like Mr Surfacer, 1000 or finer, as the primer level because it goes on thin and doesn’t obscure fine relief detail.

[Jim Starbuck]

I’ve had much better results thinning TruColor with lacquer thinner instead of acetone.
I was having tape pulling paint up as well when using acetone. My thoughts on that are that the solvent was flashing off before biting into the primer. I haven’t had any problems since switching to lacquer thinner since it slows things down a bit. I was also laying down the TruColor too “dry”…spraying too far back and not laying down a wet enough coat.
I agree 100% with Bryan on using Mr Surfacer primer.

[peteski]

I suspect the problem is with the paint applied too dry.  Tru-Color paints contain acetone which will slightly attack the plastic surface, which should proved super adhesion (the paint becomes one with the plastic).

I also agree that using the dedicated thinner is a must.  Pure acetone evaporates too quickly; the other ingredients in the Tru-Color thinner slow down the evaporation allowing for better bond, and for paint to level better.

Of course contaminated surface can cause adhesion problems.  I wash all the items to be painted with warm water and dishwashing detergent.  Sometimes I also resort to 99% Isopropanol (or Naphtha) for even more thorough cleaning.

[Sokramiketes]

My experience is the same.  Tru-Color is more of a plasticized film coating than a paint coating.  Primer is a must, especially on resin kits!  You want some tooth.  And then lay it on wet and let it suck itself tight to the surface as it cures.

[robert3985]

When I wash my stripped cars/engines before painting, it's always a two-step process.  First, I use Heptane (Bestine) to get rid of any non-water-soluble oils that may be on the surface, then I wash with warm water with Dawn in it...enough Dawn to get suds, but not too much more.  This gets rid of any water-soluble contaminants as well as additional greases/oils.

I then blow off the car/engine and then let it sit for several hours to make sure all the water is gone.

I blow it off again before painting.

I rarely use primer, on either plastic or brass as getting the airbrush spray down just wet enough is the trick. 

I do use primer when I want to eliminate plastic translucency, such as on cars that are molded from white or yellow plastic such as ME UP wooden caboose bodies or Golden West CA3/CA4 UP caboose kits and I'm painted them UP Armour Yellow, but it's not to promote paint adhesion, it's just to make the bodies opaque.

In my opinion, for what it's worth, I want as few detail-hiding layers of paint as possible, and I've never had a problem with paint adhesion on either brass or plastic...without primer.

On the other hand, I haven't used Tru-Color Paints yet in scenarios that require masking, so my opinion may change after I use up my supply of Scalecoat II.

Cheerio!
Bob Gilmore

[peteski]

I guess I should add that while I have not used Tru-Color a lot, in the past I have extensively used Accu-Paint which seems to be similar (or identical) to Tru-Color paints.  I still have decent stash of Accu Paints, and still use them. Yes, whatever this stuff is (I was told Accu Paint was an ink not paint, whatever the difference is).  It is definitely not a typical paint. When using Accu Paint, I have almost always applied them over bare plastic and did not experience paint lifting, even when masking.

As it has already been mentioned, the plastic needs to be totally clean and free of any oily or greasy residue.  Painting is done in wet coats.  Humidity should also be low so there is no water condensations while  the cool spray of paint settles on the model's surface.  Bob probably doesn't' have that problem, but living in New England, the weather gets quite humid in the summer.  No painting when the dew point starts starts creeping up into the 60s (irrespective of the air temperature).

[robert3985]

...As it has already been mentioned, the plastic needs to be totally clean and free of any oily or greasy residue.  Painting is done in wet coats.  Humidity should also be low so there is no water condensations while  the cool spray of paint settles on the model's surface.  Bob probably doesn't' have that problem, but living in New England, the weather gets quite humid in the summer.  No painting when the dew point starts starts creeping up into the 60s (irrespective of the air temperature).

Although I live in Utah, I paint at all times of the year and the humidity changes, such as when it's snowing outside or when there's a thunderstorm rolling through.  I learned from hard experience that I need water traps directly off of my compressor and inline on my airbrush hoses.  Yeah, maybe two water traps may be overkill, but I don't have water spattering problems since I've been doing that (for the past 40 or so years).

Minimally, anybody who is airbrushing using a compressor (C02 isn't going to have inline condensation problems), should make sure the tank is drained and use (my favorite) a Paasche Inline Moisture Trap on your hose...

Photo (1) - Paasche Inline Moisture Trap:


Cheerio!
Bob Gilmore

[nickelplate759]

I'm in bone-dry Arizona, so humidity isn't much of a problem.  I do drain my compressor after every use, and have an in-line filter, but I'm confident that moisture isn't the culprit.

I've used Scalecoat II in the past, and never had this problem with it.  It's wonderful, albeit stinky, stuff!

I'm going to do the primer experiment (either Mr. Surfacer or Tamiya)

[peteski]

Yes, it is good to have water trap on compressed air line, but I was talking about the condensation in the spray stream, and on the model being painted.  When the air/paint mixture comes out of the airbrush nozzle (or from a spray can nozzle), the decompression caused the sprayed liquid paint to cool down. In high humidity environment that can easily condense water from the air and mix with the sprayed paint.  Also,  when the cool paint spray settles on the painted object, it cools it down. If it cools below the dew point, moisture from the air will form water droplets on the surface. That does not play well with the wet paint on the surface.

[SD452XR]

I use an adhesion promoter I pick up from the auto parts store. It’s used by painters on plastic bumpers and such.

[SD452XR]

I clean the model throughly. Use the adhesive promoter. Primer and then the paint. Allowing a good drying time between each step. Haven’t had any issues with paint lifting off.

[peteski]

In my experience I never had the need to use adhesion promoter on any of the (clean, bare) Polystyrene or ABS model shells.  Every layer of paint adds thickness to the paint coat, and in N scale my goal is to apply the paint as thin as possible.   Organic-solvent "stinky" paints have the solvents aggressive enough to bond really well to (clean, bare) bare plastic.  Water based "acrylic" paints usually need primer to improve their adhesion to bare plastic.

Adhesion promoter is usually needed (along with adding flex agent to the paint)  when painting flexible urethane parts like car bumper "skins", or on some hard to paint resin parts.