"Bad primer?" Topic

A month (or more?) ago I primered a set of WTJ plastic ships with Tamiya spray primer. They dried for over 24 hours, then went into a metal storage box. I just pulled them out of the box and found they are tacky.

I let them sit in the open air for 24 hours in case the paint needed to "re-cure" or something. Didn't help.

The primered ships are two different kinds of plastic (DLP gray and the clear stuff), so I doubt this is a case of a bad chemical reaction with the underlying material. Tamiya spray paints are formulated to adhere to plastics, so I wouldn't have guessed they'd have a problem. OTOH, obviously Tamiya chemical engineers couldn't have predicted the 3D printing materials in use today.

Other possibilities I can imagine:

• A bad can of paint
  
• An old can of paint
  
• Insufficient curing time before going into a box. Is the primer still off-gassing for more than 24 hours?
  
• Metal storage box. Maybe a more porous container would be better…?
  
• Off-gassing plastic. There are 3 types of plastic in that box, all 3D printed. Is one of them off-gassing something that degrades Tamiya primer? Tamiya paints dissolve in lacquer thinner, a high-VOC chemical bath. Do 3D plastics emit any VOCs after printing?

I'm really just guessing.

I'm also unsure how to proceed. I should probably strip and re-paint these models just to be careful, but… OMG, what a PITA. I hate doing all that work if it's unnecessary.

- Ix

Humidity? I always brush my primer on, never seems to be the right weather.

Suspect its the material not being compatible with Tamiya spray. Those sprays were designed for injection mould plastics, and you're right to doubt they've updated it for the latest materials.

With some old school resins I've found traditional spray primers don't adhere and end up tacky like this. In those cases I've stripped and re-primered with auto primer, it's thicker than model sprays, but it works.

I'm trying 3 simultaneous experiments:

• Strip and re-paint with a different primer
  
• Overspray with a different primer¹
  
• Overspray with the same primer (which contains the same solvent and should theoretically bond with the old primer), allow to cure in the air, then overspray with a different primer.

The stripping process was hard enough² that I decided I would rather just repurchase the miniatures than strip these. I've been through too many projects like this in the last few years³. I can't face another.

- Ix

Footnotes
1: Rustoleum 2x Ultracover primer, which is supposed to bond to plastic. It's worked on the other 3D plastics I've tried it on.

2: To strip the one ship, I put it in a bath of lacquer thinner in my ultrasonic jewelry cleaner. The lacquer thinner melted the plastic basket and scarred the plastic parts of the device. After all that, the ship still wasn't clean enough, so I had to use a toothbrush dipped in lacquer thinner to get the old primer more fully off. Yech.

3: I have an entire fleet of 1860s ships that was ruined by a single bad can of primer. I have dubbed it the "twice built fleet" because stripping it also destroyed most of the glue holding the pieces together, and removed a lot of the careful customizations with putty, wire, boats, hand-crafted parts, etc. I have had to repeat a good deal of the original assembly work. Some pieces (guns, boats, vents) fell off and went down the sink, and some of the ships were simply destroyed outright by bad chemical experiments. The final batch is still sitting in a bucket in the garage because I lost my will to continue. What was supposed to be a quick-and-easy 6-month project turned into one of the biggest miniatures debacles of my hobby life.

I feel your pain.

I don't have any experience with 3D printed materials, but I have used a variety of spray primers on various metals, plastics, and paper products (like carry-out boxes, which make great science fiction buildings), and I'm happy to share what little I know.

I have gotten the best results across the board when I let the painted things cure for 24 hours indoors, in a ventilated room, on a surface (like a table), exposed to the air in all directions (except, obviously, the feet or other bottom surfaces). That's true of primers, paints, and matte finish coats.

Would Simple Green harm the resin?

Lacquer thinner is best avoided for cleaning. Ultrasonics work fine with demineralised water, and if you're lucky enough to have lime free tap water. Simple Green should be fine as well.

Also, switch to gesso as your primer… ;-)

I've primed and painted 3D printed models from Shapeways. Gesso has been the most reliable primer to avoid interactions with the material.

I had the same issue with a Rustoleum product, I over sprayed it with my clear coat and that did away with the tackiness.

My experiments paid off.

It appears I misremembered what primer I applied originally. The ships I resprayed with Rustoleum primer are still tacky; the two I resprayed with Tamiya primer cured properly. I might be wrong and the Tamiya primer might get tacky over time, but I doubt it. I'm going to optimistically assume I've discovered a working primer (Tamiya) and go forward.

I tried a different stripping method using only a toothbrush dipped in lacquer thinner. This made a helluva mess (white paint spattered everywhere and coated my fingers), but was relatively easy to do, and worked well enough to repeat. Some primer is left behind in the corners that are hard to scrub, but in general the miniature is cleaned up enough to respray with a different primer coat.

The lacquer thinner doesn't appear to have affected the DLP plastic. The clear WTJ plastic is opaque white in spots, but I can't tell if that's from the lacquer thinner or from my previous acetone bath to prep/clean the surface.

- Ix

A side note: once again, Tamiya to the rescue. I have always had a dim view of their products because they're expensive and the bottle paints are difficult to brush on, but the deeper I get into plastic modeling the more Tamiya products I find there waiting for me. The Internet has actually been a great Tamiya sales tool for me; it used to be hard to find out the tricks for thinning and extending their paints, now there are forum posts and how-to videos all over the Internet.

I've the tacky issue more than once and it has always been with a Rustoleum product.

For removing paint from plastic figures I've used Lestol, hot water, and a tooth brush. Soak the figure in the Lestol for a while then scrub with the brush under hot water. Chemical strippers (even thinners) may react with the plastic. You can also try soaking in hot water and then q quick dunk into cold water. The expansion and contraction of the material should loosen it and allow the paint to pell off.

Probably late, but I find that some of the softer plastics don't like primer, but doing a wash or two over them with like Agrex Earthshade or the like usually makes the stickiness go away. I dont know why.