[Alpha67]

I have new fenders that I want to prime and paint. They have the factory semi-gloss paint/coating on them. The question I have is what level of sanding will these need for proper adhesion? (suggested grit?)
Is it a dumb idea to skip the sanding of the inside areas of the fenders? (I HATE sanding) I'd like to simply spray these surfaces if its possible

[33Willys77]

There are 2 types of adhesion - mechanical (sanded surface) or chemical. Once a coating is cured, you can no longer get a chemical adhesion. Now, there are some products that will adhere, but you are proably somewhere along the line going to take a risk of delamination and/or quality outcome. Some higher end paint systems have prodcuts your can just spray on over the ecoat, but I still would sand/scuff it. It really only takes a few minutes with a DA to sand a fender and run over the edges with a scuff pad. You can do this easy by sanding it with 180, prime (probably etch prime) and seal if necessary and apply paint. Wet on Wet, no sanding in between. This is done all the time in repair shops on a daily basis.

[swvalcon]

On most aftermarket parts I sand as good as I can and put down a coat of SPI epoxy. I just dont trust that china primer.

[Alpha67]

Thanks guys - I knew the sanding of the exterior surface was a must - was wondering if I could avoid sanding the nooks and cranny's of the inside of the fender though (will be visible/exposed on the engine compartment side

[mitmaks]

Originally Posted by Alpha67

Thanks guys - I knew the sanding of the exterior surface was a must - was wondering if I could avoid sanding the nooks and cranny's of the inside of the fender though (will be visible/exposed on the engine compartment side

Unfortunately with body prep there is no easy way out, that's why not everyone enjoys it. You do have to scotch brite all the surfaces in order for whatever you put on top of it to stock to it.
I would use maroon scotch brite (320 grit) pad and then give it 1-2 coats of epoxy (I use SPI) and wet sand that with 400-600 and you're ready for paint.
Here's some pics of fenders I did for a guy with the above process.

[Alpha67]

Originally Posted by mitmaks

wet sand that with 400-600 and you're ready for paint.

Thanks
Regarding the 2nd image from the left, I assume this part had to be re-primed prior to paint due to the burn through around the edges? Or is this burned through to another coating? (and not bare steel)

[deadbodyman]

It really depends on the car ,for instance if its a 69 camaro your restoring you would sand off all the primer and use a good quality epoxy that you trust....if its just a daily driver you would just scuff with a red pad spray some adheasion promoter and shoot it..After market sheet metal has crap primer on them, OEM sheet metal has pretty good primer so that can stay on...basicly aftermarket parts get the primer sanded of and the jambs just scuffed and sprayed with a light coat of adheasion promoter (bulldog works fine )

[mitmaks]

Originally Posted by Alpha67

Thanks
Regarding the 2nd image from the left, I assume this part had to be re-primed prior to paint due to the burn through around the edges? Or is this burned through to another coating? (and not bare steel)

These 2 fenders had black EDP primer and I scuffed them and shot 2 coats of SPI epoxy and then wetsanded. Before painting I went ahead and shot another coat of epoxy (reduced)

[deadbodyman]

Hey Mit , whats EDP mean???? I'm not up on all these terms...and why not just sand it off if you care enough to use SPI ???Personally I dont trust that primer that comes on it ... if I care enough to use SPI it comes off ,If I dont care (like a used car ) it stays on...It always so thin it sands off ez enough ,,,

[mitmaks]

Originally Posted by deadbodyman

Hey Mit , whats EDP mean???? I'm not up on all these terms...and why not just sand it off if you care enough to use SPI ???Personally I dont trust that primer that comes on it ... if I care enough to use SPI it comes off ,If I dont care (like a used car ) it stays on...It always so thin it sands off ez enough ,,,

EDP electro deposit primer, still remember this from my college days years ago
It's actually not bad primer and you can paint over it. These fenders were shipped all the way from Japan (according to the owner of the truck) and I tried wiping them with some acetone and it didn't come off easily. I just prepped them for epoxy outside and inside and then inside got some rubberized undercoating.
EDP is not as bad as some people might think and you shouldn't have problems with it later down the road. I see lots of people just slapping EDP primered parts on their cars and driving around and then months/years later I see these panels rusty. It's only meant to protect metal from rusting until you are ready to prep it and paint. It doesn't have permanent protection from elements like urethane has.
Most shops prep these panels same way, scuff, some primer and then color.
Think of it as gel coat on fiberglass panels, you don't take gel coat all the way off before you paint fiberglass panels, do you?

[mitmaks]

Originally Posted by deadbodyman

Hey Mit , whats EDP mean???? I'm not up on all these terms...and why not just sand it off if you care enough to use SPI ???Personally I dont trust that primer that comes on it ... if I care enough to use SPI it comes off ,If I dont care (like a used car ) it stays on...It always so thin it sands off ez enough ,,,

Personally I haven't seen problems with this primer, unless a rusted/flaking part came to me. Then yes, I would strip it all the way to metal and use epoxy/urethane primer.

[deadbodyman]

Originally Posted by mitmaks

EDP electro deposit primer, still remember this from my college days years ago
It's actually not bad primer and you can paint over it. These fenders were shipped all the way from Japan (according to the owner of the truck) and I tried wiping them with some acetone and it didn't come off easily. I just prepped them for epoxy outside and inside and then inside got some rubberized undercoating.
EDP is not as bad as some people might think and you shouldn't have problems with it later down the road. I see lots of people just slapping EDP primered parts on their cars and driving around and then months/years later I see these panels rusty. It's only meant to protect metal from rusting until you are ready to prep it and paint. It doesn't have permanent protection from elements like urethane has.
Most shops prep these panels same way, scuff, some primer and then color.
Think of it as gel coat on fiberglass panels, you don't take gel coat all the way off before you paint fiberglass panels, do you?

LOL. I avoid fiberglass like the plague...so I wouldnt know...But I do see what happens to an after market penel after a few months compared to a panel that was sprayed with (spi) epoxy I always figured they rusted so quick because of the sun and the primer being so thin but mainy just some crap they put on it to keep it from rusting before it got to the states...I will say this though I use a lot of aftermarket parts (crash parts for newer cars) and they sure have come a long way as far as the fit goes some of them hardly need any work to pass,..

[mitmaks]

Depends on the manufacturer, some are better than others.

[Alpha67]

Originally Posted by mitmaks

These 2 fenders had black EDP primer and I scuffed them and shot 2 coats of SPI epoxy and then wetsanded. Before painting I went ahead and shot another coat of epoxy (reduced)

ok, got it...thanks for the clarification

[Alpha67]

Originally Posted by 33Willys77

You can do this easy by sanding it with 180, prime (probably etch prime) and seal if necessary and apply paint. Wet on Wet, no sanding in between. This is done all the time in repair shops on a daily basis.

Thanks - Wet on Wet; Does this mean I can spray a sealer and immediately after spray the paint? By sealer, is this just primer, or something special?