sanding off paint with adhesion problems-down to primer or metal?

The primer seems stable, but the **** sprayed on top of it wasn't in some places that were not sanded I guess.

Can I just sand to primer, or do I need bare metal? I'm sure bare metal is best in a perfect world, but id like to avoid the extra time and expense involved where that's even possible.

also, on metal that's pockmarked by corrosion, after I clean it with naval jelly etc in my case, do I put bondo on it before etch primer or after or will high build primer over etch primer be sufficient?

I'm not 100% sold on naval jelly due to fear of adhesion problems later on, but I have to do something andd thaat pdf tech was quite nice.

Thanks in advance

 

It's best to have a nice fresh sand on bare metal for best adhesion results. Those little rust spots need to be very clean. So clean you can't tell they where there or they may haunt you later. After sanding the metal with say 150 to realy etch the metal, give it a coat thin coat of etch and prime with your top coat 2 part primer. Don't do more than one panel at a time so you can concentrate on that panel for max results. Those little rust spots need to be cleaned with those little cleaning wheels made by 3m or similar. You can get them at body shop supply houses or even places like napa or spot sand blast lightly with out swelling the spot. Prime your metal with in the hour or you should resand .

 

One of those green 3m wheels that's got plastic tongs on it? I have one of those, most amazing thing ever. I never thoughg of using it on corroded metal.

 

if your going to paint over paint that YOU KNOW has adhesion problems SAND IT TO METAL and start over i just went through this battle with my girlfriends 50 pontiac i didnt sand it all the way down ......NOW I REGRET IT VERY MUCH but it was just supposed to be a quikie weekend paint and flame job...........................not now

 

Go to a auto paint store.....if car is a driver take it let them see what to give you to use. Will save you $ in the long run

 

Strip to Metal.
The End.

VR&C.

 

If it's just lifting because of poor prep work, I've had some luck just sanding into the primer and blending in the top coat. If it's a compatibility problem then it's best to take it to bare metal. I'm a little paranoid when it comes to rust, I would sand it, use a speed blaster on the pits, then use por 15 over that, sand, primer sealer over that and top coat.

 

Why chance it? Take her down to bare naked metal.

 

Thank you, gentlemen!

The car had black primer, and it appears the last coat of primer wouldn't adhere because its flaking off as I DA it in spots.

The front fenders are from another car and look like they have a bunch of paints on them, and id bet incompatiblity problems caused the tiny dots or something chemical.

Why should anything be easy. I'll buy sandpaper in the mornig. Thanks again!

 

Tip on buying sandpaper. Get paper designed for automotive use. Cost more initialy but cheaper in the end and faster. Home depo stuff is ok for your house and drywall products. You can kill a sheet of cheapo paper in seconds. Get the good stuff. It's a lot more money but if you say it fast ,it's not so bad.
Another thing. Try not to handle the fresh metal with your hands before priming. Get oils and what ever from your hands and can create a adhesion problem. Have you ever touched a spot of bare metal and see your hand print etched, rusting where you touched it? You get this in humid conditions. Bare metal starts to oxidise with in hiours depending on humidity. I avoid priming and painting on humid days.

 

When sanding off flaking paint, you will find that you will chase the flaking edge forever in order to get it to feather out.
You will then be left with low spots where the primer, that is softer than the paint, is sanded away even if you don't go all the way through to the metal.

This will produce wavey panels in the finish paint job unless you reprimer the entire car and then block sand.

 

stip it back to bare metal,etch prime-epoxy prime-primer surfacer. use the same product line all the way thru paint, this way you know what you have. I would never spend money on color with out knowing the foundation I was painting over!

 

I have learned the hard way more than once. Old repaints will cause trouble. If your going to try to do real nice paint get, all the old stuff off. Sealer coats are only as good as the bond they are covering up. I will never us a comercial Sand Blaster again for body tin. Sand paper is plenty good but as mentioned $$$$$$ and a lot of time spent. I just did my personal 57 Ford in 09. It had factory paint, 2 repaints then I sent it to a painter that worked on top of all that ****. It didn't work out. I went back to bare metal "NOT" using sand paper or blasting. I took my Old Sioux electric Buffer (very slow speed) and used 7" Scotch bright pads. No heat, no gouging and the paint came off lime powder. I started at 8:30 in the morning and by noon I had both quarters, deck lid and doors to the point you see in the photos. You can buy the pads by the box at any auto paint supply along with the self adhisive backer. I used 5 pads to that point. Way easy, less $$$ and nothing toothed up on the sheet metal. If you try this you'll wonder why someone hadn't already mentioned it. You'll also never regret going to bare metal.
The Wizzard

 

Pist-N-Broke, I like the idea a lot, can you be more specific as to what you used? i.e. color of 3M pad and rpm of buffer?
I am just getting ready to strip the sides and ends of a 58 GMC 1/2T LB (fenders are off and ready for blocking) and am still looking for my preferred way to strip paint. Want to stay away from chemical cause the neutralization is just another extra worry and in my space blasting doesn't work. For speed I found its been 4.5 grinder with 40 grit down to the primer (3 to 4 coats incl. original) and 80g DA on down from there. I can see a slower rpm has the 'cooler' benefit.

 

My old Sioux is a 1700 rpm unit. The 3-M disc are not listed in Grit like sandpaper. I bought a box of Black and Red pads. Looks like I used 1 Black listed and Heavy Duty on the box. The Red say General Purpose. I only have 2 left so it shows I liked them best. A skim coat of filler didn't slow them down. Over all a very nice job. The hardest part was hanging on to that old Iron Sioux. It was a good work out.
The Wizzard

 

Thanks, Pist n Broke!

I have an old metal buffer and a box of sanding pads, I'll give that a shot!