Odd paint behavior

[DaveB]

Working on a NH state police car using the Lindberg Charger kit, I primed the hood and laid down 3 coats of metallic Champaign for the roof and hood but I ended up with this odd line on the hood. There is another line in the same spot on the underside. The metallic flake looks like it's laying differently on either side of the line. This line was visible in the plastic before I painted as well. I thought the Tamiya primer might take care of it but apparently not. Best way to fix it?

 

[bobss396]

It looks like a swirl in the plastic ghosted. I would block sand it out and prime it again. 

[peteski]

That line shows how the liquid plastic flowed during the injection process (from gates on both sides of the hood, and met in the middle.  The problem is that your primer/paint combo is "hot" enough to have the solvents affect the plastic, exposing the joint line.  Similar to what happens when you sand  down scripts on a car body, then paint it, and the ghosts of the scripts show up.  This was never a problem when using mild hobby paints.  Not sure how  to guarantee that it will not show up after sanding and repainting.  Maybe spray a barrier coat of the Killz shellac-based clear?

[Ace-Garageguy]

I don't know what paint you used, but I can tell for sure it is way too hot to use on styrene model parts, particularly the soft junk they're using now.

"Too hot" means the solvents are too aggressive, and will attack the plastic leading to the very obvious "crazing" you have here. 

Properly applied paint, even metallics shot "dry", isn't gritty looking like that (and clear will just make it look worse).

Tamiya primer is simply not sufficiently solvent resistant to provide a barrier in a situation like this, so the paint solvent soaks through it and etches the underlying plastic...crazing...and accentuates mold-flow defects like your line in the process.

To get an acceptable finish on that, all the crazing needs to be sanded out, the part needs to be re-primed with something that will provide a barrier, and you should really use a much less aggressive, less "hot" color.

The hood below ghosted several times after I removed the peak and sanded out some divots, and shot it in hot Duplicolor primer.

I fixed it by sanding, re-primering, sanding, re-primering, sanding, re-primering until I'd killed it.

The final topcoat of hot Duplicolor looks like this. Note that it's not gritty or textured.

 

One more thing...we constantly harp on the importance of TESTING the materials you want to use BEFORE committing to painting a model...and develop some understanding of various techniques.

Painting models has become considerably more challenging of late, what with much less solvent-resistant styrene being used, so testing has become critical if you want results you can be proud of.

You will also find things work much better if you stay within one product line (like Tamiya) that's made for models all the way through one job.

Mixing and matching hardware store/big box store or automotive paints is a recipe for disaster until you really know what you're doing...and even then TEST FIRST is the golden rule.

 

 

 

 

Edited May 28, 2024 by Ace-Garageguy

[DaveB]

The paint is Montana Cans Montana Effect Champaign, and I just realized I did a stupid. I completely forgot I also bought a can of their primer to use on plastic before using their paint, I was never planning to use the Tamiya primer. Stuff happens when you get old...

[DaveB]

Been busy over the summer but got some model time in. I stripped the hood with 90% alcohol, lightly sanded then spayed with the Montana primer then sprayed with the metallic champagne, problem solved.

[DaveB]

Final product after a couple coats of floor polish, good enough for government work.

[OldNYJim]

Looks like the line is still visible though, plus you can see some crazing of the plastic in the center of the hood.

If you’re happy, cool!  Rock on!  But a less ‘hot’ paint than Montana will help prevent that and give even better results…

[DaveB]

10 hours ago, CabDriver said:

Looks like the line is still visible though, plus you can see some crazing of the plastic in the center of the hood.

If you’re happy, cool!  Rock on!  But a less ‘hot’ paint than Montana will help prevent that and give even better results…

For some reason pics of this paint look like garbage no matter how I take them, looks far better in person, could be the metallic. Montana was the only brand I could find this color in, the Montana primer seems to work well.

Edited September 8, 2024 by DaveB

[NOBLNG]

5 hours ago, DaveB said:

For some reason pics of this paint look like garbage no matter how I take them, looks far better in person, could be the metallic. Montana was the only brand I could find this color in, the Montana primer seems to work well.

I had the same thing with Tamiya light gunmetal paint.? It doesn’t photograph well at all. The pics I took with my iPhone looked grainier and far worse than the ones I took with the iPad. Both look worse than it does in person.

[Ace-Garageguy]

It's a vast improvement over what you had before, so be happy.