Beginner filler/primer sanding questions..

[trent]

As of today, car is expoyied and built up with high build primer. PPG products. I usually search search, but it wasn't working well on the old board. I have a couple questions.

Can filler be applied to a high build primer? OR does the high build need to be sanded down to epoxy and then filler applied, then high build?

Dura-block sanding blocks are great. However, in making a panel perfectly straight, I have slightly gone though the high build to the epoxy. I have not however gone into the metal. Is this a problem? The epoxy is dark grey and the high build is light grey. Will this create a color problem when I lay the base? Do I need to lift the entire panel a little bit with the high build?

Thanks for the help!

TTT

[trent]

As of today, car is expoyied and built up with high build primer. PPG products. I usually search search, but it wasn't working well on the old board. I have a couple questions.

Can filler be applied to a high build primer? OR does the high build need to be sanded down to epoxy and then filler applied, then high build?

Dura-block sanding blocks are great. However, in making a panel perfectly straight, I have slightly gone though the high build to the epoxy. I have not however gone into the metal. Is this a problem? The epoxy is dark grey and the high build is light grey. Will this create a color problem when I lay the base? Do I need to lift the entire panel a little bit with the high build?

Thanks for the help!

TTT

Bump. Need some help please. Sanded through the high build and epoxy onto the metal on another section trying to be careful. Confused on what I need to do to fix this.

Thanks guys

[Phil V]

Trent, its not a big deal. Just spray on some more 2-k high build primer. It won't hurt if you end up with a small patch of bare metal here and there before you spray more 2-K primer. In a case like that I never spray more epoxy over the bare spots before putting on more 2-K primer. The 2-K primer will stick just fine to those bare metal spots.

[Phil V]

Forgot to add that in most cases its OK to put a little polyester spot putty on top of 2-K primer but its not a good idea to load bondo over 2-k primer.
I personally am against applying bondo to anything but bare metal. And that bae metal has to be sanded with a 24 or 36 grit grinder disc to put grinder marks in that metal which gives the bonbo something to "grab on to" (a good mechanical bond).

[Jason]

You don't want to put a 2k primer over bare metal. 2k primers are a urethane product, urethane attracts moisture as it cures, get moisture on bare metal and get corrosion. Definetly want to put a self etch or epoxy over bare metal.

[Phil V]

Jason, 2-K primer or for that matter any isocyanate based chemcially hardener paint/primer does not attract moisture. The isocyanates based hardeners will react to moisture in the air which actually aids in the cross linking of the urethane molecules. It doesn't draw moisture and it certainly doesn't provide moistue to bare metal which will/could cause rust.


Reaction of isocyanate with water

O
||
R - N = C = O + H - O - H ----> R - N - C - O - H ----> R - N - H
| |
H H
isocyanate water unstable product amine

[65Comet]

Good to know Phil. I've been touching up metal spots from my blocking with a spray of epoxy through a preval sprayer,,,just so I don;t have to clean my gun for such a small spray...

thanks

g

[trent]

Great information, thank you fellas!

[Jim Streib]

I too had problems with sanding some spots and breaking through to metal or came close to it and instead of just applying more material over it what I was seeing was a high spot. I got out my body hammers and lightly tapped the area down and then put more material over it. If I had it knocked in enough the spot would not reappear. If it did reappear, it got another tap ot two to knock it down but not hard enough to where I had a low spot that needed to be filled. I don't do body work all day long and tapping the metal on mine was one of those things I might have gotten right the first time and not had the high spot reappear or I would have to tap it some more. I have a hard time feeling the very minor high and low spots and it's one of those things some learn.

Something I'm doing on my 74 is sanding it down to a 400 grit and then seeing what kind of reflection I'm getting off of the surface to see if I need to do any more work. If you see or feel a flaw in the primer stage it will only be magnified once the finish paint is on it.

http://image40.webshots.com/40/6/44/...3GSvDVg_ph.jpg

http://image14.webshots.com/14/6/47/...1bMScuc_ph.jpg

Jim

[Len]

The later steps of the filling, sanding, filling, sanding process should be guide coated. Most folks that haven't restored many cars won't have the touch to get the surface perfect without SEEING the highs and lows. Use a guide coat during the later stages of the "bondo" work right through the filler primer stage. Guide coating during these later stages of filling will help eliminate the highs and lows that make the job look less than perfect.

For wet sanding use a spray-on guide coat like SEM and for dry sanding use 3M Dry Guide Coat. It's amazing how they will improve your work. After one guide coat lesson my helper is finially producing top quality work.

[Phil V]

Len, Got your grey primer on there a little heavy didn't You ? hehehe. It was for visual effect right ?

[Len]

Glad it was primer!