Another paint/rust question...

[spudly13]

So i had my Z car sand/media blasted probally 2 years ago, since its been sitting in a heated garage, ive done a lot of metal work since, and didnt seal it after blasting as i found out i "should" have... there is some surface rust from air tools and what not in places, and im sure dirty finger prints everywhere..

 

So my problem is after searching the interent for a couple days ive found that there is so much controversy over what to do as far as prep and products to use, everyone has there own opinion and its hard to say whos right when they all argure with everyone else.

 

Im getting to the point where i need to lay down some primer so i can sweep some bondo... my question is what should i do to prep the metal short of media blasting again? sand the rust down, wipe it all down with (????) and spray it with an epoxy primer? obviously the idea here is im worried about rust in the future, any products to avoid? any input would be appreciated!

 

Thanks

 

[Skib]

sand it and properly seal it.

[DeviousPsycho620]

I just came from my local auto paint store. I was told to sand all the rust away. Epoxy prime it. Do a sandable primer next. Sand and wipe with a tack cloth. Then thin the epoxy primer down and spray it again. Then paint.

[DanielC]

I let parts sit in bare metal too.  it is not ideal, but no big deal.

Sand rust away.  You know that.  Here is what you may not realize.  Bare steel will start to oxidize almost immediately, unless you live line in the Atacama Desert.   I am using PPG DPLF, epoxy primer, and PPG  1791/1792 Wash primer.  The instructions for both say to prime carbon steel immediately after cleaning.

 

I have found a small abrasive impregnated nylon bristle cup brush at Ace hardware, with a 1/4 shank, that you can chuck in a drill.  

Even though there is no visible surface rust on the steel, after running this brush over the steel, it is definitely brighter.  it will also remove light rust.

I mix the primer.  Then I use the brush, and blow off with the same air I supply to the paint gun.  Then prime.    

I should use a cleaner on the metal after the brush, but I don't.  I need to do that at least once to see what comes off the metal.

Do not use a wire brush to do this.  The steel wire just "burnishes" the metal, kind of line polishing it.  The abrasive in the nylon brush leaves enough tooth for good paint adhesion.

[spudly13]

Sweet, ill to go down to ace and grab a couple of those discs, after i spray with a good epoxy primer i can lay my bodywork on top of that correct? thats my understanding anyway, just double checking!

 

What would you wipe it down with? acetone, mineral spirts, thinner?

[DanielC]

In the PPG line, I would use a DX 330.  You can also use the same reducer as you use in the primer on top of the metal.

Use two clean paper lint free towels.  Get one wet with cleaner, and wipe a small area you are cleaning.  Before the cleaner dries or evaporates, wipe it off with the clean dry paper towel.  If you see any residue on the dry towel, you need to rewet the surface, and wipe again with another dry clean paper towel.  

[K_trip]

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[spudly13]

isnt a sandblasted surface technically already etched?

[K_trip]

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[DanielC]

I am going to suggest, but not argue, that putting body filler on bare metal is not good.

This is why.

Most body fillers are a polyester based resin.  This resin absorbs water from the air.   You are applying some water on bare metal, whern you apply the mixed body filler on the metal.   Water makes bare steel rust.

Polyester resin, once set up is porous.  It will continue to absorb water, unless sealed.

 

The preferred method, I believe is to properly clean the bare steel, then epoxy prime the metal.  Allow the time required for the primer to cure, for using under body filler.

With PPG DPLF that time is 1 hour, at 70 degrees, with one coat of primer, and overnight with two coats of primer.  Your primer probably is different, read the data sheet that you got with your primer.

[K_trip]

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[spudly13]

the controversy is why i finally just decided to post for myself and try and find people with painting experience to guide me in the right direction, that body filler test is interesting, but i have heard previously that bondo direct to metal can cause rust...

 

So heres what im thinking, (let me know what you think)  scuff everything with an abrasive disc, clean with a cleaning agent of some form, spray with a good epoxy primer, lay body work on top of that? the other thing is fiberglass and resin, will those bond alright to the primer? i have a little glass work to do with my fender flares as well.

 

im still looking into metal etching, i recall at one point somebody saying that the phosphoric acid can mess up certain primers? if this isnt the case ill etch it before i clean it just as another step to insure a good paint job in the end.

[DanielC]

I have been working on some fenders...

Those are just the left fenders, and since the picture was taken, I got another pair from a 521 that was at Sherwood P-n-P late last year.

After removing paint, sometimes several layers, and plastic fillers, I have found rust on a lot of them, under the plastic filler, when the filler was direct to metal.

 

I am not saying that every time you apply plastic filler direct to metal, it is going to rust, but the evidence I have seen indicates to me that there is a good probability it will.  Every time, no.

I have no way of knowing if the metal was treated with a phosphoric acid before the filler.  My eyes are not that good. 

I was taught to use phosphoric acid treatment to "kill rust".  Some primers actually recommend it, others do not.  YOU HAVE TO READ THE DATA SHEETS OF THE MATERIAL YOU ARE USING!  I have seen a trend to not recommending phosphoric acid treatments, mainly due to the problems caused by the acid not being "neutralized"  or cleaned off well enough.

 

Another issue is this.  Simplified, there are two kinds of body work, on cars.  There is a get it done, out the door, beat the flat rate, and get on the next insurance job type of work, expected to last the reasonable life of the car, and then there is restoration work, where beating a flat rate does not matter.

 

There are sometimes I take the long road.  I also take shortcuts, and hope they do not come back to haunt me.

Last week I did some spraying of primer on fenders.  Ask almost anybody, and it was too cold.  But with heat lamps heating parts up, and moving some things out into direct sunlight, I think I may have gotten away with it.

 

I also did a fiberglass and resin repair in the roof of Ratsun.  The roof was looking like this.

I started out with coarse steel wool, and phosporic acid, and removed a lot of the rust.  Scrub it, let it sit, scrub some more, wipe off with paper towels, and Dawn and water, in a spray bottle.  Then I sanded it, 80 grit.  While sanding it, I also did a fair amount of hammer and dolly work on the roof.

The next step, PPG 1791/1792, a rust preventive wash primer, then DPLF.  That was allowed to cure.

 

The rectangle thing in the middle of the roof is a steel plate, that had some caulk, to block a bunch of holes from a spotlight.

 

To cover the several holes the plate was blocking off, I used fiberglass with marine epoxy, from Tap Plastics, and then sanded that, and then more DPLF, and finally plastic filler.  I worked the filler down and then used a high build primer, after spotting the bare metal again with DPLF.

 

Then paint. 

The picture is a lot better than the roof actually is, there are a few dents still in it, and also a lot of sand scratches, but that is how I learn.  

[dattoboy70]

best thing to do is epoxy primer bare metal then do body work over that. just my 2 cents