So this is a 3rdgen urethane bumper, I painted it (very badly with electric spray gun back in HS about 16 years ago). Time has come to address the bodywork. The paint I sprayed actually held up VERY well, no real chipping, no peeling, I just started and decided to sand the bumper down with 100grit first, but there were some paint cracks that are not disappearing..

Do I sand all the way down to urethane where the cracks are?

Can I go with rougher paper like 40 grit? I'm doing it by hand for now.. I do have a DA I can go with if needed.

Are these cracks in the paint? primer? or urethane itself? they seem pretty deep

or do I buy a flexible urethane filler, there are bunch of good ones now and simply do a skim coat over these?

pic (please zoom in, the pic is huge) https://www.dropbox.com/sc/3zd1npqlg...BVePgwcqNTBDia

thank you in advance for any help.

 

You're gonna have to take it down past the paint I'm sure. Sand until it's gone, if you still see cracks once you get to the poly then it's in the bumper. I'm not sure what the best grit is for the bumper but there's a lot of good info on autobody101.com I'd search there.

 

Those look to just be cracks in the paint.


100 grit is pretty coarse. I don't think I would go rougher than that.

You will 100% need to go threw all layers of paint down to the urethane.

Hopefully the cracks are just in the paint, if not replace the bumper or use a Good quality filler. This means NO Bondo!!! lol

Generally you want to use a 220 grit pad to knock the paint down. Then work your way up (220,400,600)

 

220 to knock paint down? wow, that's gonna take me forever! lol

I see lot of youtube vids where they go 50/100/150/220.. I Just don't know if that's ok on urethane too

 

guess it's time to break out the DA, I shot a LOT of paint on this car many years ago heh

 

Urethane reacts differently than metal. You don't want to scar that bumper up. I wouldn't risk damaging the bumper just to save time. I almost never go more than 180 unless I am stripping the body down to bare metal.

Better safe than sorry!

 

100 is fine to start but once you get close to the bare plastic you will want to finish it off with nothing rougher than 180- once its bare hit it with 320- after its bare wash it with soap and water and let dry then use a bare plastic cleaner and a plastic adhesion promoter. once the adhesion promoter is on and has tacked up you can spray a high build urethane primer on it. with bumpers this flexible its best to use a flex additive. do 2-3 medium wet coats of primer with the flex additive. once the primer has dried its best to wet sand with a 400 grit by hand so you dont burn the edges down to bare plastic. once its all been prepped wash it again with soap and water, then use a wax and grease remover designed for pre paint and then go have at it with your base coat clear coat or your single stage paint. I would steer clear from any of the dupli-color garbage. its low end and old technology that doesnt last for crap. you can go to and auto body supply store or even oreileys and get nason products which is dupont/axaltas economy line and get the all of the materials for probably only 25-30% more than duplicolor but they will hold up much better.

 

Agreed!

 

As far as what grit to sand to before primer you're best to read the spec sheet on primer. My DuPont 2K even wants different grits for different surfaces ie fiberglass vs urethane vs paint vs body filler. I believe for Urethane it wants to see 400 grit but some surfaces it wants as course as 220.

I use 120 grit to start with and try to stop as soon as I hit the original primer or break through the color coat. As much as you can use the original paint as your primer surfacer and you'll save time and money.

You'll need to sand all the way through the cracks and then at least feather it out. My personal preference would be to repaint the whole bumper if the paint is older than 10 years. If it's just a spot repair don't bother with the flex additive but use the flex additive if you repaint the whole thing. The flex additive will only be used to help avoid cracks when moving it, installing it and wet sand/polishing it.