Painting over Original Paint
 

I am currently working on a game plan to paint my truck. I will be starting with the firewall in order to get some mechanical tasks complete in the engine bay before working on the cab and doors/hood.

I plan to paint over the original paint in this area, using a single stage urethane from a high quality manufacturer that is TBD (recommendations?). Im going with the OE medium blue that the truck was built with.

The OE paint is in great shape on 97% of the firewall, with exception to some mild surface rust near the heater box, brake booster, electrical connection gasket, etc.

What is the proper way to remove this light rust prior to scuffing and shooting primer and paint? I also have some very mild areas where I scratched the paint to bare metal while chipping away at the original grommet sealant.

I plan to use this same method for my interior and dash as it is also in 97% "good shape". Truck will be a high quality driver, perfection is not necessary but I am doing my best as an average joe. The rest of the truck will be going to bare metal as it is not in the same condition.

Thanks in advance!

Re: Painting over Original Paint
 

I would strip it to bare metal if I were you. The original paint was lacquer and you can have problems painting over it. I learned the hard way. Urethane paint can chemically bite into whatever it is painted over, it makes it hold better., but it can have a bad reaction also. You might get lucky if you use a good sealer first. Acrylic paint is safer going over other paints.

Re: Painting over Original Paint
 

I've had some luck shooting touch ups and blending in over the original paint (it's enamel, not lacquer) by using "metal prep" and a scothbrite or fine steel wool to clean up light rust and de-gloss the orig paint surface. I cannot recommend this as a long lasting, professional approach, but I personally have not had any failures even after 20 plus years of indifferent care.
I think most would argue for a sealer between new and old paint, but I'm here to say I've cut corners and gotten away without it.
You'd be smart to test first way down low or on a spare part or something.

Re: Painting over Original Paint
 

Thanks for the replies. I would much rather prefer the sealer approach to taking this area down to bare steel. I have plenty of that to do and was counting on having the firewall and interior go a little quicker than the rest of the truck if possible.

What type of sealer would be recommended? And would it make any sense to use a different type of paint in these areas in order to have a more durable paint job?

Any others with comments or experience here?

Re: Painting over Original Paint
 

I would sand the paint with 320 being in the engine department degrees it well. SPI epoxy would be the best thing to use as your sealer. Single stage urethane is Your best choice

Re: Painting over Original Paint
 

I don't know what year each company changed from lacquer to enamel, but it was around the late 60's early 70's. I have painted over old paint with no problems may times before too, but I have had problems also. It's easier to strip it now than after you spend all the time doing bodywork and then have a problem. If you clean it well and put the first coat of sealer on light you may be fine. Then wait a little before you spray a second coat, longer if it's colder.

Re: Painting over Original Paint
 

If you have old paint and rust what I generally do is I seal it first with the permanent rust sealer / surfacer. The reason being that's what it was designed to do. Not available in body shops so you never hear of it. This primer has a very mild solvent like mineral spirits so it doesn't lift the paint below it. This is the only type of primer the blocks both moisture and oxygen and solvents. apply 2 coats of silver and then an epoxy or urethane surfacing primer over that block it and paint it . You can use this over any type of primer and paint and use any kind of primer and paint over it
This is the most anti-corrosive system you can create.

Re: Painting over Original Paint
 

Here is actual practical experience for your consideration. Original factory enamel paint with light surface rust. I used Metal Prep solution, which if I'm not mistaken is mild phosphoric acid in combination with wetting agents and other proprietary ingredients, applied to let stand, then gently worked around with very mild abrasive (my personal favorite is #0000 steel wool) until rust disappears. Rinse well and dry thoroughly. Others have bragged about how CLR solution does magic, but I find that stuff is weak childs play by comparison.
Next is buff and wax, or paint and polish to suit whatever your needs are.
This has worked well for me on many orig paint projects where saving lightly rusty factory paint was important. I would NOT recommend this for metallic paints, as the acid solution attacks the metal particles within the finish more aggressively than in solid color finishes. Please experiment at your own risk.
I have successfully sprayed single stage acrylic enamel directly over this very surface on several occasions without ill effects. If you choose a more aggressive topcoat, perhaps a good pre-sealer would be an option.

Before and after Metal Prep on factory paint:

Re: Painting over Original Paint
 

One more before and after surface rust abatement.

Re: Painting over Original Paint
 

Thanks everyone for the comments. I am a little delayed in response here due to the holidays.

Combining response to various comments above:

- Is it generally agreed that an epoxy primer is an acceptable "sealer" for this application? I've seen "sealers" available from some paint manufacturers, how does this differ from an epoxy primer or other product?

-If I am going through the trouble of adding an additional sealer/epoxy layer in this process, I would consider it just as simple to wire wheel the rusty areas to bare steel before the epoxy and avoid the trouble or "unknown" of using acid/metal prep on these areas before epoxy/primer/paint. Since there may be a slight difference in surface texture of these areas, it would probably sand easy enough with the high build primer. If not, its a firewall and not a show truck... Any thoughts to this idea?

-If I proceed with the idea above and remove the rust anyway, the issue is more paint/chemical compatibility over the OE paint. Does anyone have a negative experience with shooting epoxy/high build/ single stage urethane over OE paint that is in great shape?

:b69::tim:

Re: Painting over Original Paint
 

Permanent rust sealers such as Mastercoat and Rust Bullet are encapsulators. They're designed to go over old paint and rust. These types of encapsulants put a airtight ,moisture tight seal over the surface. They will do upwards of 8000 hours of salt spray, epoxy around 700. You can seal it with anything you like ,how long it lasts is another story .

Re: Painting over Original Paint
 

It is just my humble opinion that once you have broken through the factory applied paint and primer/sealer, then you have opened the door for all kinds of adhesion and/or reaction issues. The edge between new and old will be the most trouble as far as potential lifting or wrinkling goes and also presents a bit of work to sand out or feather properly. It's way easier in my mind to leave the killer factory stuff intact and just prep the surface to accept a new coat of color. I think this would be especially true in an enclosed area where years of sun and weather have not thinned the paint film much. I could be flat wrong on this, but to me just the idea of "encapsulating" a problem gives me the willies.

Re: Painting over Original Paint
 

Remove the surface rust with 80 grit, spot prime the bare metal areas with a coat of SPI epoxy and apply a full coat over the whole thing. If you need to smooth things out a bit apply 2 coats of SPI’s high build, block it down to 400 minimum, seal with a reduced coat of epoxy and apply your topcoat.