Why did my primer crack?

[JE123]

I put primer on my Ferrari F50 today. I used the same type of primer that I always use. But today, I noticed that it cracked in some places on the body. What causes this? I was thinking that it might have been caused by me leaving the pieces outside in a box for about 15 minutes, before I brought them inside.

[x3m]

what's underneath the primer? what did u do before primering?

[JE123]

There was just plastic underneath.

[LauBak]

Was the primer a thick layer?Or did you spray a new layer before the bottom layer was completely dry?

[Ranger_X]

yah, that's what I was thinking, layered on too thick. Just sand it down and try again

[JE123]

I sprayed one layer, waited about 15 minutes then sprayed another.

[LauBak]

I don't sure if my prediction is correct but perhaps 15 mins is not enough for a layer of paints/primer to dry completely. The covered new layer maybe crack due to the different degree of pulling back as the base layer.

[x3m]

u should spray adhesion promoter on the plastic before the primer. try Bulldog Adhesion promoter, you can buy it in spray cans.

[x3m]

try to lay a wet and thick primer in one stroke so that u don't have to primer it the second time. let it dry. sand it. if u think it still needs another layer of primer then this is where i should primer it again

[AccordTypeR]

Tamiya suggested appox. 20 min for drying....
But I think if you want complete dry....i thikn at least you need to wait a day.....

[astroracer]

Okay.... Lets step back and take a breath...
x3m... your suggestions are well taken but, for model cars, you are going in the wrong direction. Less is better. The least amount of primer will do the job and do the job well. The problem with spraying on "thick wet coats" is you lose all of the detail. Door lines get buried, window trim gets buried, emblems, etc. and you run the chance of what JE123 has experienced. A lot of shrinkage which leads to cracks in the paint and a generally poor paint job.
I know many of you don't have an airbrush but excellant results can be had using "bombs". If I am doing major body work I will use a bomb to throw on some primer that I know will get mostly sanded off. For finish priming and getting ready for top coat I always use an airbrush. I spray the primer right from the spray can into the airbrush jar, then spray the car with very light coats. You can do this with a spray can but you need to keep the model out of the heaviest portion of the spray pattern. Use the outside edges of the spray to coat your model and do it with a distance of 10 to 12 inches between the model and the spray head. Keep the coats light and allow 10 to 15 minutes between coats. Doing this will give you will have a good base for final sanding and paint.
Mark

[nikanora]

Another thing you guys might want to try doing is washing the plastic and let it completely air dry before you prime/paint it. I know many people suggest this with gundam models (my past modeling hobby) because they would come with type of oil them(it comes from when the model plastic was being made) and it sometimes repels paints, causing them to crack. Just a suggestion.

Nika

[primera man]

How hot was the day??
If the paint has dried to quick in hot conditions it can cause this problem if the paint was sprayed on to thick.

Also...did you sand the model first?
I like to give the model a complete sand over to give the primmer something to stick to.

[x3m]

ok my bad i though u guys are talking bout real car =D, yeah just a light primer will do and don't forget the adhesion promoter just a light coat of it will do.

[Kosmonaut]

perhaps you left greasy fingerprints on the model that prevented the primer from sticking properly and it cracked
did you use a tack cloth? i once used a tack cloth that left a waxy residue all over the model and screwed up the paint
i say sand down the cracks, wash the car with detergent, and continue on with paint
personally i would not reprime the body
too much primer will obscure details
good quality paint like tamia paint for example, will stick to the sanded body