Sand fiberglass before micro?

alpinelakespilot2000 · #1 10-02-2007, 05:29 PM

In laying up a rear canopy skirt I forgot to use peel ply on top of the fiberglass plies. Before filling with microballoons, do I need to scuff up the cured fiberglass with sandpaper or do microballoons adhere without any scuffing? Thanks.

az_gila · #2 10-02-2007, 05:45 PM

I would.. it will help the filler get a good mechanical bond... I would use 80 grit or coarser...

gil A

4kilo · #3 10-02-2007, 08:37 PM

Also, if you have used West System epoxy or something similar for the lay-up, there will be a waxy-feeling "blush" left on the surface. If you clean this off with simple soap and water the sanding will be much easier.

If you try to paint or glue anything over the amine blush, it won't stick at all.

Pat

az_gila · #4 10-02-2007, 11:34 PM

As Pat says.... this is from the WEST instruction page...

Cured epoxy-Amine blush can appear as a wax-like film on cured epoxy surfaces. It is a by-product of the curing process and may be more noticeable in cool, moist conditions. Amine blush can clog sandpaper and inhibit subsequent bonding, but it can easily be removed. It's a good idea to assume it has formed on any cured epoxy surface.

To remove the blush, wash the surface with clean water (not solvent) and an abrasive pad, such as Scotch-brite(TM) 7447 General Purpose Hand Pads. Dry the surface with paper towels to remove the dissolved blush before it dries on the surface. Sand any remaining glossy areas with 80-grit sandpaper. Wet-sanding will also remove the amine blush. If a release fabric is applied over the surface of fresh epoxy, amine blush will be removed when the release fabric is peeled from the cured epoxy and no additional sanding is required.

Epoxy surfaces that have not fully cured may be bonded to or coated with epoxy without washing or sanding. Before applying coatings other than epoxy (paints, bottom paints, varnishes, gelcoats, etc.), allow epoxy surfaces to cure fully, then wash and sand.

The entire WEST system manual is here... well worth a careful read...

http://www.westsystem.com/webpages/userinfo/manual/

I admit to skipping the wash stage, but for small items such as a tail tip or tail fairing, the sandpaper clogging is not too bad if you use non-clog paper...

But most of the time I used a peel ply, which removes the blush as it is removed...
Regular sand paper doesn't last long...

gil A

RV7Guy · #5 10-03-2007, 08:35 AM

One, clean the area with soap and water, then carefully wipe the area down with acetone or lacquer thinner. At minimum, denatured alcohol.

It is important that you don't sand any of the waxes into the work.

Two, before adding any additional epoxy or cloth I always sand to insure a good bond. Always clean after sanding.

The above apply to any work with fiberglass regardless of the type, polyesther or epoxy resins.

alpinelakespilot2000 · #6 10-03-2007, 12:33 PM

Thanks for the elaborations from all. I do have the West Systems manual and read it quickly, but didn't realize that the amine blush could be almost invisible. If I have it, it is not at all noticeable, but I will defintely wash first before getting the sandpaper out. Rather, next time I'll remember to use that piece of peel-ply I so diligently cut out beforehand but forgot to use!

Thanks again for the tips.

kevinh · #7 10-03-2007, 02:29 PM

When I made my skirts, I used an old trick I used on a fiberglass airplane:

After laying up the last layer of glass just pour micro over the layup (you _really_ need to know that you won't be adding more glass - so the skirts are a good for this). Capillary action will pull epoxy into a little bit of the micro - then once dry you brush off the excess micro and start sanding. You'll find that the weave is almost filled and the micro is very easy to sand/fill.

YMMV, but it worked for me.

alpinelakespilot2000 · #8 10-03-2007, 03:19 PM

One additional question... I've noticed that others often do sanding and filling and then shoot a coat of primer to help identify remaining low spots. They then sand some more, fill some more, shoot some more primer, etc. The West system manual suggests avoiding using any self-etching primer because it apparently does not work well with the epoxy. Given that, what would be a good, readily available primer to use for this purpose?

As always, thanks for the education on these fiberglass topics. Based on suggestions from a previous thread, I layed up a 2nd skirt that turned out much better than the original.

jsharkey · #9 10-03-2007, 04:21 PM

The general rule when preparing composites for bonding or paint is to (1) remove any surface films such as mold release or the byproducts of polymerization by first detergent and/or solvent washing - followed by (2) abrasion and then (3) a final solvent clean. Repeat steps 2 & 3 until you get a "water break free" surface, i.e. water doesn't from droplets on the surface but speads as a wet film. Abrading before washing can grind adhesion prohibiting material into the surface of the part from where it will leach into a bond line or paint layer causing adhesion degredation.

Jim Sharkey
(General Manager, Vermont Composites, Inc. - day job!)

DanH · #10 10-03-2007, 05:04 PM

Steve, there are a couple ways to go with filling. I assume you're using microbubbles and epoxy for fill.

The best way: Always try to apply enough micro mix to catch all possible low spots with the first application. Too much is better, and it isn't that hard to rasp off the excess with an 80 grit block.

You want one coat because small differences in the ratio of bubbles to epoxy will change the sanding hardness. If you sand out a first coat, find you still have a low spot, then fill it with a different micro/epoxy ratio, it becomes difficult to get a really flat surface with further sanding. The two areas cut at different rates. The result is a wavy surface.

You can't fix waves in micro with more micro, but you can shoot an epoxy primer and compatable primer/filler. I previously wrote about sealing the raw glass and micro surface with epoxy and then spraying PPG DPLF and K36. Block the K36 to get rid of the waves.

If you must hunt for low spots and you can't easily see them, dust the surface with ordinary cheap primer or flat black. Dust it, don't coat it. The paint is only a sanding guide; you only want enough to barely see it. Block sand, which will remove the paint dust from the high spots and leave the low spots marked. You can micro the low spots without removing the dust coat. Or you can circle them with an ordinary lead pencil and wipe out the dust coat with some MEK.

BTW, I think water washing raw glasswork is a bit overboard for amine blush. The underlying cause is slow cure in the presence or water vapor or CO2. If you do your work in a cool damp spot you'll get blush. Do your glass work in a warm, dry place and you won't get much. Serious 80-120 grit sanding or peel ply will make it a non-issue. I do wet sand epoxy seal coats, but that's mostly because epoxy sands better wet with fine grits. You get an amine wash as a bonus.