Why does my primer keep cracking?

[Guest]

Hello all, I have started the process of re paining my bike. I have so far primed the front fender and the side covers. The front fender (which is ABS plastic) came out great but the side covers look great right after I shoot the primer but after a couple of days display hairline cracks in them. Why? The prep work for the fender and the side covers has been the same but I'm not getting that problem with the fender which in my opinion would be more prone to cracking since it is more flexible. BTW I am doing all the work in my basement which is a constant 20 degrees Celsius with moderate humidity.

Thanks,

Raz

[Sie Motorräder]

You need to use a primer that is formulated for plastic.

I have used rattle can products from duplicolor and there is a pre-treatment called an adhesion promoter. I think this is used prior to primer or is a primer in itself.

You also need to wetsand before applying paint in most cases.

[Nessism]

It's not a plastic issue, it's an old paint and/or contamination issue.

Rattle can paints (which I assume you are using) don't like to lay down over certain types of old paint. I suspect the old paint is being attacked by the solvents in the primer you are using thus causing the cracking you are noticing.

I suggest you sand down the old finish to get to the base plastic or at least to the old Suzuki base paint. Wash really well with simple green and let dry. When you do your primer coats, do light dusting coats, several of them. Don't do heavy, wet coats since there is too much solvent in these type of layers. Several light dusting coats will eventually build up to the thickness you desire. Wet sand with 600 and then do your top coat.

Good luck and hope this helps.

[razooki]

Thanks Ed, I'll try it.

[Sie Motorräder]

I just checked and old plastic rear fender I did some years back, Sure enough, I had to go to the bare plastic to get proper results.

Its strange that the primer and paint I used for the metal front fender did not work on the back.

I eventually bought a NOS rear so I now have a poorly color matched rear in case of accidents.