Using Bondo as a filler

[Ratdog]

Moderator Note: Splitting this discussion out from this thread: https://www.canardzone.com/forums/topic/21972-sales-ive-seen/page/37/?tab=comments#comment-79320

On 9/16/2021 at 11:23 AM, Kent Ashton said:

Yup, Vari just above marked "sold".  Interesting that it appeared to have the old RAF-recommended black carbon-rich UV primer to which paint did not stick very well.  I had my own experience with that and it likely means a strip-down, refill, refinish, and repaint.  Lots of work.  I wonder if the buyer knew that.  Surprise!  😞

What about fillers i know some were filled with bondo which doesn't hold up very well . How can you tell if bondo was used and is the only cure to sand it off . 

[Kent Ashton]

8 hours ago, Ratdog said:

 How can you tell if bondo was used and is the only cure to sand it off .

Bondo is heavy but I would not be alarmed by a little of it.  Certainly wouldn't want to use it for extensive filling though.  I don't know any good way to check.  Most guys stick to epoxy+micro or West (epoxy) and West 410 Microlite.   https://www.westsystem.com/the-105-system/fillers/

The West filler is very expensive in the small containers.  Best to buy it in a 4 lb box.   

[slk23]

In addition to being heavy Bondo also shrinks.  IMHO it's unacceptable except for perhaps pinhole filling.

Recently I've been using the 635 Thin Epoxy Resin System from uscomposites.com.  Combined with micro-balloons it sands like the West Systems epoxy but is much less expensive.

[Jon Matcho]

12 hours ago, slk23 said:

Recently I've been using the 635 Thin Epoxy Resin System from uscomposites.com.

I assume you're using this for fill only? Just so others reading this are aware, there are only a few epoxy systems suitable for structural usage. This 635 system is UScomposites.com's own, and there's no information available regarding its mechanicals to compare against the other epoxy systems that are used, and it's an unproven system in terms. @slk23 you may be the first.

All I could find on it is that UScomposites.com does say that it's for structural use (boats, kayaks?) and this MSDS sheet.

[slk23]

3 hours ago, Jon Matcho said:

I assume you're using this for fill only? Just so others reading this are aware, there are only a few epoxy systems suitable for structural usage. This 635 system is UScomposites.com's own, and there's no information available regarding its mechanicals to compare against the other epoxy systems that are used, and it's an unproven system in terms. @slk23 you may be the first.

All I could find on it is that UScomposites.com does say that it's for structural use (boats, kayaks?) and this MSDS sheet.

Jon,

I was replying to the two posts just preceding mine which discussed fillers. I also specifically referred to using the UScomposites epoxy with micro-balloons. I think the context was clear. 
 

For purposes other than filling/contouring I always use E-Z Poxy. 

[Jon Matcho]

On 9/18/2021 at 1:00 PM, slk23 said:

I think the context was clear.

It wasn't entirely clear to me at least. Sure, the context included filler but nothing was as clear as this for example: Do not use the 635 Thin Epoxy Resin System from uscomposites.com for anything structural.

With that said, I would imagine this 635 system would be much better than Bondo for filling. Purists might want for only an "approved" epoxy system, but the only at-risk item here would be paint falling off.