Epoxy Question

For West Systems, at least, it's slightly different, but only because of the fillers. There are other variants for other purposes, though. You start with the resin, which is common to all applications. Then you pick a hardener, either the makes-glue type or the makes-paint type. We want the makes-glue type. Then you add fillers if needed - none for glassing, either structural fiber or filletting blend.

Yes, no and maybe.

In the case of, say, West System, it can be used for all the aforementioned. Sandability and absorption are keys. Other epoxies can be used for structural adhesion (NHP, et al.) but don't do as well for other tasks (lamination, etc.).

I tack fins to a motor mount with 5 min NHP (minimal bond), then do a fiberglass lap fin root-to-fin root over that section of motor mount with West fast. I do fin fillets with EITHER West OR NHP WITH 404 West filler for sandability and shapability. West is "better" but the 5 min is quick and gives more ability to make complex form around fins (allows many fill and sand layers to be done in less sessions).

Come on Gene, you only have 11 days to finish that rocket of yours!! Hope to see it fly on Aug 7-8 at the METRA launch :-)

Rich K

aforementioned.

For gluing and filling, you may want to thicken up the standard thin "laminating/coating" resin with some of the available powdered fillers. These come in several varieties, including "milled fiber" (which makes a dense sticky material, good for strong glue joints), "microballons", which produce a lightweight easily-sandable material, and "fumed silica" or "Cab-o-sil", which tends to gel the liquid mixture so that it stays where you spread it.

You can also get commercial pre-mixed epoxy products along the same line, Aeropoxy 6209 and 6279 "liquid adhesive" and "paste adhesive" (similar to adding various quantities of milled fiber and fumed silica to liquid epoxy for gluing use), or PolyFiber "Superfil" and Aeropoxy "Light Filler", which resemble lightweight microballoon-loaded mixtures: good for fillets and similar smoothing/fairing uses.

-dave w

Glassing epoxy is much thinner than the traditional epoxies. You can add assorted fillers and thickeners to glassing epoxy to use it for other purposes (which is what the manufacturers of the common stuff do for you). Any epoxy that is mixed with a 50:50 ratio probably has some sort of filler added to one or both parts already. You can not use the thick stuff for glassing.

IMHO any epoxy sold in a hardware store, general merchandise store, grocery store, etc is not worth using. Also true for CA from these sources. And again IMHO 5 minute epoxy is worthless. If I need something done fast, I use CA, otherwise I use medium to long cure epoxy.

Usable stuff (both epoxy and CA) can be found in good hobby shops. The "house brand" is from Bob Smith and quite usable, as are the typical hobby brands.

All I buy any more are the resins, and I add whatever filler is appropriate when necessary. They tend to use odd mix ratios, from 2:1 to 7:3 to 5:1. Typical brands are West, SystemThree, and Raka.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

In case the question relates to this [and you well may know this already but I thought I'd say it anyway] remember that fiberglassing resins that you find at hardware storesin for autobody work or boat repairs are not epoxies [unless they specifically say so]. Typically autobody glass repairs use polyester resins catalyzed with MEKP, where the cure rate can be modified by changing the amount of catalyst used (within reason). Boat makers may use polyester resins as well, however many use vinyl ester resins which use the same catalytic cure system but are preferred for their better overall structural properties. Epoxies are better yet. Cost, not surprisingly, follows in proportion.

Laminating epoxies are typically neat epoxy resin with added reactive and/or non-reactive diluents and sometimes wetting agents. Structural epoxies are filled epoxy resins, typically a mineral filler and often some fumed silica to reduce settling and increase thixotropy. Some contain tougheners. They can use various curative systems depending on the application or the manufacturer.

Thus in a pinch you can conjure up a passable structural epoxy by adding a filler such as finely powdered silica and ensuring you maintain the correct resin to curative ratio as specified by the manufacturer, but without some thixotroping agent (i.e. Cab-O-Sil which is fumed silica) you may have a tough time getting fillets to stay put.

Mike D.

I *HATE* polyester resin. Stinks, doesn't always cure right, makes a huge mess.

Thixotropic additives are your friend!

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

I've often used a bit of everything for fillets that are intended as reinforcement as well as fairings: a bit of milled fiber (chopped fiber is too coarse) for reinforcement, a bit of microballons for lightness and sandability, and a bit of fumed silica to gell the whole thing a bit.

-dave w

Bob Smith epoxy had the lowest strength of any epoxy I tested. (We're talking orders of magnitude here.) It's junk.

Doc

Doc, but can you REALLY tell us how you feel? :-)>

Your post is blather.