Flour as a filler in epoxy and polyurethane

[rjcullis]

Hannu Vitaria recommends using regular bread flour as a filler. I have experimented with it in polyurethane and found the polyurethane/flour mix to be stronger than the wood it is bonded to (exterior grade fir plywood). I am now experimenting using epoxy/flour filler with some fiberglass boat repairs as well as a filler on a 1958 16 foot Cabin Cruiser we are repairing in my school shop. Has anyone out there used this combination and do you have any advice?

[Dan McCosh]

Hannu Vitaria recommends using regular bread flour as a filler. I have experimented with it in polyurethane and found the polyurethane/flour mix to be stronger than the wood it is bonded to (exterior grade fir plywood). I am now experimenting using epoxy/flour filler with some fiberglass boat repairs as well as a filler on a 1958 16 foot Cabin Cruiser we are repairing in my school shop. Has anyone out there used this combination and do you have any advice?

Only that talc used to be used as a filler, and it was lousy--absorbing water and eventually swelling. Seems as if flour would be similar.

[boylesboats]

I haven't try it yet...
Would you think it would get moldy if water/moisture find its way in there?

[westinghouse]

Wood flour. Yes.
Grain flour. No.

(I'm assuming the boat is going back in the water - for a floor display any flour will do)

[JimConlin]

Do you feel lucky?

http://affordablehousinginstitute.org/blogs/us/wp-content/uploads/imagesclint-do-you-feel-lucky-small.jpg

AFAIK, nobody has done any reliable lab testing on that material in boatbuilding applications. Why take chances?

[pipefitter]

Yep, I've used it for some epoxy rich, micro-fairing putty above waterline. Goes on well but it sure loves to eat sandpaper. No problems so far. Flour is a plant material much like wood flour is a plant material. Once it's saturated with epoxy, what is going to want to eat it? At any rate, after sanding I sealed it with straight epoxy. Anything that is left porous and wet will mildew pretty much.

http://home.earthlink.net/~tigmaster41/sitebuildercontent/sitebuilderpictures/104-0578b_img.jpg

[SMARTINSEN]

Why?

In relative terms, fumed silica in its various flavors is inexpensive.

BTW, nice avatar, Westinghouse.

[Thorne]

I'm sure you can use nearly any solid, non-volatile substance as filler in epoxy -- cat litter, dirt, you name it. But clearly defining "success" and "failure" are much harder to do, and what may be OK for a CDX ply & sheetrock screw boat drysailed for a year may not be good at all for a larger wooden boat that lives in the water.

The epoxy companies have spent many years and lots of dollars on just this question -- no need to re-invent the wheel here, folks!

[westinghouse]

Why?

In relative terms, fumed silica in its various flavors is inexpensive.
I agree.

BTW, nice avatar, Westinghouse.

Hehe - you too.
And Thorne, I do like your hat.

[Cuyahoga Chuck]

The thixatropic fillers commonly used in epoxy are all rated for strength.
The strongest are the micro-fibers like chopped fiberglass or cotton.
In the middle is WOOD flour. Handy because you can make batches in many different shades for color matching. And it's quite cheap to buy.
The weakest are substances like microballoons which are good for fairing because they make sanding easier.
Cabosil is fumed silica and makes for a hard mix. Haven't tried it yet.
I tend to use wood flour for filleting and West Systems 403 fibers for scarfing or applications where I want good strength. 403 is very inexpensive and a can lasts a long time.
I concur. Don't try to re-invent the wheel.

[pipefitter]

I used wheat flour in a pinch for a slick fairing putty above waterline. Was the last couple spots I wasn't satisfied with before priming the hull with epoxy. Wasn't worth waiting a week for the 'proper' material. I had the day to git'r done or it was going to put me off for a week further into the approaching rainy season. Wood flour is a biodegradable powder as would be wheat flour. Unless one steps up from a fine powder into a more fibrous and interlocking granule such as coarse sawdust, the strength ratios are not going to be much, if at all markedly different in a structural sense. In most situations concerning fillers, such as those used for main structural fillets in a hull, it's usually fortified with multi-axial fiberglass tape layers with redundant overkill which all but cancels out the structural integrity of the fillet itself beyond a form to alleviate sharp and non-conformable inside corners.

As far as reinventing the wheel, that's exactly what stitch and tape construction has been doing, after it had been determined for centuries that chines and keelsons made of wood had already proven structurally durable for the task.

Would I use wheat flour throughout a hull to replace the more acceptable fillers? Probably not but if I had a situation where I needed to repair a spot instead of buying a whole kit for a nickel job, I could get away with using it and probably know how to compensate for whatever it may lack otherwise. I can tell you this much, it makes for a very tough putty high in abrasion and impact resistance.

[soba]

Hmm...

wood flour and wheat (grain) flour may seem similar in that they are both "plant" based and "biodegradable," however one is cellulose and the other is largely starch.

Wheat flours do contain some fibre, which is why they are a good source of dietary fibre...but they are not wood.

Now, I'm not sure all this matters. Epoxy largely encapsulates filler material, and short-term absorption by filler material is moot. However, most hydrophilic materials will swell and swell and keep getting larger over time. Epoxy is not reputed to be a permanent barrier for water...just a good one. Wood flour, if it does get exposed to water via epoxy penetration, will swell and shrink if the water is allowed to escape. I think grain flour would be the same...but the rates might be different. Dunno.

The wood flour and grain flour both have the drawback of being composed of irregular particles, and will not form a thixotropic fluid...in fact there is some evidence it may form a rheopectic material in something as viscous as epoxy.
I wonder how well kaolin would work?

[john welsford]

Dont forget that hard baking flour is used as a bulking agent in the Phenol Resorcinol glues used in exterior and marine plywoods. It works fine there, but is at a fairly low proportion of the total resin mix.

JohnW

[ModH-28]

I'm a grist miller and would be happy to mill some flour at any consistency you choose. Our stones were built in the 1840's and originated in France. I wonder if that helps in a wood boat situation. Merry Christmas!

[Captain Blight]

Our stones were built in the 1840'sI'm pretty sure your stones were around in some form or another before the 1840s....

[Lew Barrett]

Where do all these ideas come from, anyway? And why would somebody want to experiment in this particular fashion with the options available. This is something I have never really understood.
Is this expected to provide a benefit, or just be an alternate?

As much as I could avoid using hydroscopic materials in my bonding agents, I would.

Soba, it was so tempting to suggest ground noodles as another option, but I have decided not to. :D
I thought your reasoning quite sound, by the way.

Added: Back years ago we used to use Bondo to fair topside planks with. You could even buy it at most chandlers, although that is clearly no longer the case. I took a lot of heat, and rightfully, for mentioning it and everyone was fast on me pointing out that Bondo is hydroscopic. I have not forgotten the sting of giving bad advice here, and even though I fancy myself experienced (if not always fully informed) I think that my own view is now very well fixed. Nothing hydroscopic in my bonding or fairing agents.

[Jay Greer]

Why bother?
Jay