Scratchbuilding......bondo problems

[Sixx]

First of all, here's to a healthy and happy 2008 to everyone!!!

I'm having some problems with bondo these days, I don't know if my product is just too old or what!?

I built a rear spoiler out of styrene sheet and rod, then applied bondo to the seams. After it was good and dry, I waited 24 hours before I sanded it down. When I painted it with metallizers paint, it developed a few pin holes.

I sanded the primer off and re-filled the holes (with bondo) and waited for it to dry again, then re-sanded and it seemed to sink into the once filled holes almost like joint compound when it shrinks.

My questions are:

1* how do I know if my bondo is still useable

2* is bondo a chemical process or a heating process, meaning it adheres better when dry?

3* When scratchbuilding what two part puttys do you guys like best?

4* Is bondo the correct material to fill holes and imperfections with or is there a better substitute? or possibly is it to harsh for platics?

My builds have gotten so much better thanks to all of you guys and your help! Take it easy!

Sixx

[MPWR]

I'll try my best with this....

I've never used Bondo, as I've always been happy enough with two part epoxy putties.

1. If you mix some of the resin and the hardener and it hardens over the advertised period of time, I'd say it works.

2. From my understanding, Bondo is a catalized polyester resin, similar to fiberglass resin (just stiffer). It's cured by an exothermic chemical reaction, and will work best without water contamination. It also emits lotsa nasty vapors while curing, and probably isn't great for your skin.

3. I like two (equal) part type epoxy putties. Tamiya's is great, but there are other good alternatives. I've had good luck with this stuff, too. The nice thing about epoxy putty (often marketed as 'plumber's putty') is that it is completely compatible with water. You can add water to it to soften the mixture and make it easier to shape. It will definately dry when wet- it will even dry underwater. It's also fume free as it cures, and is fully sandable/primable/paintable when hardened.

4. Bondo may work fine for some people/applications. But I think it'd probably be worth trying epoxy putty instead.

[Sixx]

Thank You for your reply. I've never even considered " plumbers putty "!! Couple of other questions, I read alot about, evercoat-eurosoft polyester putty in Scale Auto magazine. I guess its kinda like bondo but better?! Any thoughts on that? Also have you used Aves 2-part epoxy putty? I know the figure modelers use that quite a bit, I've got some, never used it yet. Thanks again for your help!

[Joe Blyth]

I've never used Bondo either, but I second MPWR's suggestion on two-part epoxy. I use Milliput brand myself. There are several styles availible, I use this one: http://www.squadron.com/ItemDetails.asp?item=VMP0003

[MPWR]

I would shy away from catalyzed polyester. It just doesn't offer any advantages in this application over epoxy putty- plus it's unnecessarily toxic and nasty. Epoxy putty just does the job too well to bother with polyester.

I've not heard of Aves, but if it's a two part putty and the figure guys like it, I'd consider that a decent recommendation. Epoxy putty for figure work will likely have the same ideal characteristics that a good putty for doing body kits will have.

[Sixx]

Thanks for the great ideas! I agree that the smell of bondo is horrible and does stink up the whole house! I'm gonna head down to my hobby shop and see what I can find.
It's pretty sad to see everything going to R/C cars these days! not much left for us plastic builders!

[Joe Blyth]

Originally Posted by Sixx

It's pretty sad to see everything going to R/C cars these days! not much left for us plastic builders!

I agree, I do almost all of my model-related shopping online now. I think the last thing I bought in an actual store was a kit at Hobby Lobby a few months ago.

[mulsannemike]

Originally Posted by MPWR

I'll try my best with this....


3. I like two (equal) part type epoxy putties. Tamiya's is great, but there are other good alternatives. I've had good luck with this stuff, too. The nice thing about epoxy putty (often marketed as 'plumber's putty') is that it is completely compatible with water. You can add water to it to soften the mixture and make it easier to shape. It will definately dry when wet- it will even dry underwater. It's also fume free as it cures, and is fully sandable/primable/paintable when hardened.

^^^^^^^
How easily does this product sand? Use Bondo as a reference if possible.

[MPWR]

Well as I've said I've never really used Bondo- but comparing it to other catalyzed polyesters I've used I'd definitely say that the epoxy putty sands as well or better. I'd compare it to sanding a cellulose body putty, except that it holds together better and isn't crumbly like cellulose putty. It sands very, very nicely.

[mulsannemike]

Originally Posted by MPWR

Well as I've said I've never really used Bondo- but comparing it to other catalyzed polyesters I've used I'd definitely say that the epoxy putty sands as well or better. I'd compare it to sanding a cellulose body putty, except that it holds together better and isn't crumbly like cellulose putty. It sands very, very nicely.

I was just concerned about the "steel hard cure" claim. As everyone knows, it's miserable work sanding something that's more akin to cement on the hardness scale! I'm not necessarily a big fan of the polyester fillers either giving their long term suseptibility to shrinkage, but that's typically what I use out of convenience.

[MPWR]

Originally Posted by mulsannemike

I was just concerned about the "steel hard cure" claim. As everyone knows, it's miserable work sanding something that's more akin to cement on the hardness scale!

Not a problem. I've been using the A+B tan for years (in part because it lasts that long), and it's a perfect hardness for sanding. Hardened it is easily workable with sandpaper, sanding sticks, files, knifes, razor saws, etc. Much easier to work than steel.

[mulsannemike]

Originally Posted by MPWR

Not a problem. I've been using the A+B tan for years (in part because it lasts that long), and it's a perfect hardness for sanding. Hardened it is easily workable with sandpaper, sanding sticks, files, knifes, razor saws, etc. Much easier to work than steel.

What about pinholing? Naturally that's an issue with the polyester fillers are they're more akin to liquid in their natural state and take in air bubbles when you mix the filler and hardener.