Tip: Wheelpant Seam

The builder's manual has you fiddling with sandpaper on a stick and the like to straighten the flanged half of a wheelpant seam. Very tedious. Try this instead.

First the nose section half of the seam; no flange so it is easy. Set it on a flat surface, hold a pencil flat to the surface to draw a line around it, then trim back to the line with a big 60 grit rasp block. Tape a sheet of 120 to the table top to finish lap the edge dead straight.

Now remove the dust and get some vinyl tape about 3/4" wide. Run a strip around the seam edge with half the width of the tape on the outside surface, then fold the tape over so it wraps around the edge. You now have a no-stick edge.

Scuff the gloss off the flange face on the rear wheelpant half. Fasten the halves together with clecos in the future nutplate locations. Mix your favorite epoxy, then add one part flox, one part cabosil, and one part microballoons as necessary to form a thick paste. Paint a little pure epoxy into the open seam, then wipe in the paste to fill the seam entirely, plus a little.

When cured, use a vixen file to cut the filler flush to the surface, remove the clecos, and pop the halves apart. Just a few minutes with some 80 grit on a stick will tidy up the new seam edge. The result is a tight, straight seam.

Cool info, question please

DanH said:

Mix your favorite epoxy, then add one part flox, one part cabosil, and one part microballoons as necessary to form a thick paste.

Click to expand...

Si' se?or, el explain-o, por favor.

I heard of micros and flox (and used them both but not together). I have not used cabosil. I googled it, super fine filler of synthetic silicon dioxide. Cool. What are the advantages of cabosil.

I never heard of all three in one batch, if that is what you meant.

I never mixed flox or micros together either, but that is me. Thanks appreciate the info.

Flox brings toughness and a moderate increase in tensile strength. Micro just adds lightweight bulk. Cabosil is to make epoxy or epoxy putty thixotropic.

A non-structural filler paste using just flox would be coarse and wouldn't blade into a thin crack very well. Adding some micro improves smoothness and workability, although it does weaken the end product a bit. The cabosil prevents the putty from flowing out of the crack before it can cure. The combination makes a filler putty that blades nice and stay put, then cuts and sands well.

That is such a clever idea that it almost makes me want to go make a new set of wheelpants Dan.....note that I said "almost"

I love simple and elegant solutions - I did it a hard way!

Paul

Thanks Paul.

Everyone, allow me to put extra emphasis on the part about "scuff the gloss off the flange face". You can't be sure of a good bond to a cured gloss epoxy surface; you must open the surface.

You can sand, the usual method; 60 to 100 grit is about right. My personal favorite for a small part is to bead blast, because it is fast.

Painting a very light coat of pure epoxy on the fresh surface prior to putty application is the second step in assuring that the cured putty isn't gonna pop off as a chunk some years from now.

Good goo

DanH said:

Flox brings toughness..........................

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danke gracias grazie merci, Thanks

Dan,

You really need a seam gap of ~.050 to allow for paint build-up. As long as the line is straight and gap even it looks fine. Same requirement around all the cowl edges, too.

One thing I found is the necessity of using shims to even the surfaces across the seam. There were some places where the front half was .06 and the flange depth .12, so you'd get a nasty step at the seam.

Thanks for the additives explanation. Gravity was not my friend while the filler cured.

John Siebold
Boise, ID

<<need a seam gap of ~.050 to allow for paint build-up.>>

You mean 0.005"? 1-1/4 millimeters (.050") would be some mighty thick paint. 5 mils is typical for mainline domestic cars. The upscale stuff will be closer to 7 mils.

Right now the gap equals the thicknesss of the vinyl tape.

<<One thing I found is the necessity of using shims to even the surfaces>>

Yep, me too, as many as three plies of 9 oz crowfoot.

Tape thickness....

DanH said:

<<need a seam gap of ~.050 to allow for paint build-up.>>

You mean 0.005"? 1-1/4 millimeters (.050") would be some mighty thick paint. 5 mils is typical for mainline domestic cars. The upscale stuff will be closer to 7 mils.

Right now the gap equals the thickness of the vinyl tape.

<<One thing I found is the necessity of using shims to even the surfaces>>

Yep, me too, as many as three plies of 9 oz crowfoot.

Click to expand...

Good Vinyl tape is about 5 mills thick....

http://www3.3m.com/catalog/us/en001...ytapes_3_0/command_AbcPageHandler/output_html

Good electrical tape is about 7 mills thick....

http://products3.3m.com/catalog/us/...actors_3_0/command_AbcPageHandler/output_html

Perhaps two layers of tape would be appropriate for paint clearance, since there are two layers of paint involved...

gil in Tucson

Guys,

I mean .050. These fiberglass parts will need lots of high build primer before you even get to whatever paint system you use. There are two surfaces approaching each other, so the gap gets closed up quite fast.

Check with paint shops if you want confirmation. My painter (who's done a lot of RVs) insist on such a gap.

How big's the gap on all your car seams? Looks OK, doesn't it. The secret is keeping the gap even. If you aim for .010 after paint but waver .005 in your facing straight edges, your eye will see the 50% variation. Ugly. .005 isn't noticeable if the average gap is larger.

John Siebold

I hear you John. A lot of the composite guys strive for perfectly even 1/16" wide gaps (after paint) on cover plates, seams, canard junctions, etc. Looks nice. They are usually backed by a flange, just like our wheelpants.

You have a good point. It is unlikely that I'll leave .020" of primer on a male mold part, but I do think I will add a little gap to reduce the chance of edge chipping. Just put the nose cone back on the flat table and draw another trim line, perhaps the width of a new fineline Sharpie. I like Gil's suggestion too; use a few layers of tape when you do the fill step to get the gap you want.

The focus of this tip is how to straighten up the seam on the flanged half of the wheelpant. The no-flange half is easy to work.

Dan,

Thanks for the great post. This comes just in time because I'm just fitting a pair of Sam James wheel pants and there is quite an uneven gap along the front and back half seem.

I am ordering the epoxy, cab-o-sil, and "flocked cotton fiber" from A/C Spruce.

They don't have "microballoons" but they do have 3M glass bubbles. Is this the same thing?

Also, is the Flocked Cotton Fiber the same thing as flox?

Thanks!

Yes and yes, although microballoons also come in phenolic as well as glass. You want glass balloons.

If you're ordering from Spruce add a copy of "Moldless Composite Homebuilt Sandwich Aircraft Construction" to your order.

Thanks Dan! You know what? I think I actually have that book burried away somewhere. Might have to dig it out...