Bondo on the metal or bondo on the primer then re-prime it?

[80gs1000e]

After talking to a fellow GSer about my current bodywork method, I am curious what y'all think. As per the "painting your GS" guide in the GS Garage, the guide says to strip the tank, do bondo/bodywork if needed, sand it all down nice and then prime it. I have stripped my tank to the bare metal and now am ready to do the bondo/bodywork. But my friend says that I should 1st spray the primer (rustoleum primer), then do the bondo work and sanding, then re-prime it again. Any experience to share?

I'm leaning towards the bondo first, then prime it once I have finished sanding the bondo. Help please!

 

[Planecrazy]

I'm no paint maven, but my experience (and understanding) has always been that you want to do any filler work FIRST and then prime and paint. Primer isn't designed to be used as glue, so if you have a large patch and a layer of primer in between there's a good chance it will separate from the metal and crack or fall off.

Once the filler is applied and sanded smooth, primer will help fill the micro-gaps and provide a good uniform base for the paint to adhere to. That's the way I understand it.

Good Luck,
Steve 8)

 

[KEITH KRAUSE]

About 30 years ago, I took a course in auto body repair. You put the body filler on first, then the primer.

 

[80gs1000e]

Thank you both for the input. I shall proceed as originally planned. Bondo first, sand it smooth, then primer! Pics coming soon.... 8)

 

[KennyJ]

Not always. Some primers work really well under the mud. If you are using rustoleum, yes, prime after bodywork. If you are using an epoxy primer, or a self etching primer such as DP40 or DP40X, then the mud goes on top of the primer.

When doing a really nice, long lasting job, I use the DP40 (a Ditzler product) then bodywork, then a last coat of DP40. The DP40 will not let moisture get to the metal, thus no rust forming under the mud to flake it off (the most common problem with mud/bondo).

I use Martin Senor products almost exclusively, but when it comes to bare metal, nothing beats the old DP line.

Kenny

 

[80gs1000e]

Well.....I sanded the tail and side panels and got ready to do the bondo on the tank. The bondo I got has a hardener that you add to it, mix it, and then apply. But this stuff sets up in like 4 minutes (as per the instructions) and I couldn't get it on there and shaped in time.....so I panicked and ground it off and back down to the metal. It just was hardening too fast. It also got really hot....?

Can I add bondo to bondo? If I need to add more, do I have to let the stuff I put on first dry all the way, then sand and add? Or can I just gloop more on there? ..........

What kind of bondo do you guys recommend? This stuff got hard too fast!

 

[KennyJ]

Mud (bondo) instructions are very generic. You can vary the amount of hardener to make it dry slower or faster. Try a few small mix batches to see how fast they dry. I like about 5 to 10 minutes, but that is pretty quick. If you have a lot to do, slower is better. Use about 2/3rds what you are currently using for hardener and try again.

To add to what you currently have on there, just scuff the gooey surface with some 40 or 80 grit and apply a little more. I would suggest rough shaping what you currently have on the tank before adding more.

Nice thing about mud, you can keep sanding it off and trying again!
Kenny

 

[Planecrazy]

If you are using an epoxy primer, or a self etching primer such as DP40 or DP40X, then the mud goes on top of the primer.

The DP40 will not let moisture get to the metal, thus no rust forming under the mud to flake it off (the most common problem with mud/bondo).

Kenny

OK, Kenny, riddle me this ... if the DP40 seals out moisture when sprayed on the metal, then why wouldn't it do exactly the same thing when sprayed over the mud? Hypothetically, you get the exact same result regardless of what layer the DP is located at...

I'm not challenging you on this, I'm trying to understand the rationale. Does the manufacturer specifically advise spraying DP under mud? I would think that my prior concern (lack of adhesion) would outweigh going this way unless the manufacturer specifically states that their product is strong enough to hold the mud to the metal. Look forward to your response.

Thanks,
Steve 8)

 

[KennyJ]

Yes you are correct, it should do the same either way. Under the mud is preferable if you are going to do either. Why I do it both under and over is when you are sanding the mud, you will break throught he surrounding primer, thus I should reprime the bare metal, so I just go over the mud.

I have witnessed many a rusted car shot with DP40 and even 5 years later no rust has broken through. The epoxy is a sealant, thus slowing the oxidation process dramatically.

When you wet sand your primer, have you ever broken through to the mud? If so, I'll bet that mud retained a little bit of moisture, which will start the oxidation process and eventually cracked mud or worse yet, flake off.

Prepped correctly, mud can be pretty thick and last a long time. Sure you only want to use the minimum possible, but sometimes....

Kenny

 

[Planecrazy]

Ok, I understand your rationale regarding not wanting to break the primer seal, but what about the adhesion question? Is the primer rated for use under the filler? I realize that if it's a shallow patch there shouldn't be a problem, but if it's a thicker patch I would still be concerned with proper adhesion...

Steve 8)

 

[KennyJ]

Yes, it is common practice to use mud over an epoxy primer, most notably DP40 (and its derivatives, DP40X and the other DP's).

Kenny

 

[Planecrazy]

Thanks for the clarification, Kenny! Well, Dom, now it looks like you have more choice then you thought you did... I'm looking forward to seeing the result when you get it out for a ride!

Enjoy,
Steve 8)

 

[80gs1000e]

Cool. I hope to be through with it as soon as possible. I got this nice blue metal flake paint that is close to the original color so we'll see, I just want it to be done! I'm taking my time and doing it right, just hoping that the insane humidity goes away so I can paint.