Hey all, I started paint testing today on my extra wheel fender and came across a problem.
The paint bubbled right away around the spots where it had been sanded down to bare plastic.
Is this caused by needing to sand the whole fender down to bare plastic or is this my inability to properly use a spray can?

You did prime and sand and prime and sand some more didn't you?

I had issues painting plastic once. Not exactly the same but similar. I had to use adhesion promoter, then primer, more primer, sand, paint, stand back and admire.

EDIT: I should also mention, I am by no means an expert.

No paint expert here, but painted my fair share of cars with my dad when I was a kid. He painted professionally. Armor all is a painters enemy. It penetrates to the metal or plastic and causes the paint to bubble. The only way to resolve this is to sand to the bare plastic or metal. Use a sealer on the bare metal or plastic. A water borne sealer. then prime it. Then paint it. Hope this helps.

It didn't bubble at all when I sprayed the primer, it only bubbled once I threw on the red paint. Could it be a need for more curing time and a sanding between primer and paint?

Quote from: m_melen on March 01, 2013, 05:14:46 PM
It didn't bubble at all when I sprayed the primer, it only bubbled once I threw on the red paint. Could it be a need for more curing time and a sanding between primer and paint?

Yes

Also wipe it down with paint thinner be for painting to clean off any oils etc (just touching with your fingers will leave oil).

Looks to me there was some oil/dirt in those area's or those area's where sanded to metal and not preped with mud and primer (low area )

Edit: A good guy to ask paint related questions to is hprauto1 on youtube. The guy is one heck of a painter and could probably tell you exactly what caused that in the images.

http://www.youtube.com/user/hprauto1

Somewhere I had put some quotes from the book "How to paint your Motorcycle". It has a chapter on what can go wrong.

What happened here is a result of the paint solvents trapped below the hard upper surface. Those slowly evaporate as paint cures and that's why paint settles down into every nook and cranny after a week or two and actually seems to get thinner.
When you sanded, you broke through the hard surface and exposed the softer under layers.

The new paint going on dissolved the softer under layer where it was exposed with the feathered sanding.

This could have been prevented if the paint was fully cured or new layers of primer were applied to re-seal all the exposed edges of the various layers.

I urge you to get the book "How to Paint Your Motorcycle" by JoAnn Bortles from Amazon. The chapter on "What can go wrong" is worth the price of the book.


This type of thing can occur over the entire surface. This is where "flash time" comes in. When paint is applied, the under layers are soft as the surface dries and hardens. I'm sure you've felt paint that seemed dry but a little pressure moved a slab over a soft substrate like a sheet of ice on a pond.

If within the flash time, the surface is not dry and hard. This will allow the solvents in new paint to pass through and everything becomes one.
If after the paint has cured (could be a week or more), new paint won't dissolve anything and will hopefully stick to the hard surface.
If new paint applied after the flash time - when the surface is hard and what's under it is still soft - the solvents in the new paint will soften the hard layer. when that happens, the layer softens and expands producing all the wrinkles (like wrinkle-coat paint!) only here you don't want the wrinkles.

I've done it wrong - only to see the whole surface start to wrinkle or just spots that were still soft underneath the hard later. I've wasted paint and had to re-sand a belly pan or headlight cowl. Once I had to do it twice because cracks in the fiberglass were holding onto the paint solvents. New paint made the cracks wrinkle. So I waited a month before sanding and continuing with the next layers. Drying booths with heat lamps - that's what the big boys do to dry and cure the paint.

Quote from: wayne242 on March 01, 2013, 05:17:11 PM
Also wipe it down with paint thinner be for painting to clean off any oils etc (just touching with your fingers will leave oil).

Looks to me there was some oil/dirt in those area's or those area's where sanded to metal and not preped with mud and primer (low area )

+1

Quote from: adidasguy on March 01, 2013, 05:19:14 PM
I urge you to get the book "How to Paint Your Motorcycle" by JoAnn Bortles from Amazon. The chapter on "What can go wrong" is worth the price of the book.

You can read (most of) this book for free on google books.

Linky:
http://books.google.com/books?id=IjkTcSYd24AC&printsec=frontcover#v=onepage&q&f=false

Quote from: PatheticPuma on March 01, 2013, 06:19:12 PM

Quote from: adidasguy on March 01, 2013, 05:19:14 PM
I urge you to get the book "How to Paint Your Motorcycle" by JoAnn Bortles from Amazon. The chapter on "What can go wrong" is worth the price of the book.

You can read (most of) this book for free on google books.

Linky:
http://books.google.com/books?id=IjkTcSYd24AC&printsec=frontcover#v=onepage&q&f=false

The pages you get to preview can change when you go in. It doesn't necessarily have contiguous pages nor the pages you want.
Best to pay the $22 and buy it so you have the complete book.

Be cautious abut wiping paint down with paint thinner. It, too, can lift up the paint where you've sanded if not careful. There are pre-wipes and cleaners that limit the possibility of lifting up fresh paint. Tac cloths are another good thing for dust and debris.

I paint for a living....Sand, Prime/Seal, Recoat, Enjoy 

Thanks for all the advice gents, I think I have the information I need. I'll update in a couple days when it's been repainted and we'll see how she turns out

I went to schooling for body work and painting. Putting all the info together should work just fine. Sand it, clean it, prime, sand, paint, clear coat. You dont always need to sand primer but it makes adhesion better. When i painted i used what we called wax and grease remover to clean the surfaces and used a tack rag to make sure no dust/dirt is there.
Take your time and have fun, it should come out fine. Just be sure you dont get too close, runs in paint, primer, or clear coat just add more work. Good luck!!!