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Prep for glass

Old 02-10-2008, 11:39 PM
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Default Prep for glass

I'm going to start practicing laying down fiber glass in prep for my next plane. I'm used to going down to 400 or even 600 grit to prep for film, seems like I shouldn't have to go that far if I'm doing glass & resin. How far do you all go?
Old 02-11-2008, 12:01 AM
John Sohm
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Default RE: Prep for glass

220 is about as far as I ever go with balsa wood, especially if I'm glassing.

Then if I use epoxy resin, I'll give it a light going over with 100 just to get the bumps off. When it's primed, I hit that with 150 first time then 220 and 320 at the end. When I give it the first coat, I'll hit any runs that might have occurred with 320. After that I don't sand but I will use 000 steel wool to get any ridges off the markings and actually buff it up a little (I use satin finishes).

If I use polyester resin, I actually start with something like 60 grit, the polyester finish is harder than the epoxy. Give it a second (flow) coat and then use 60 to get the surface level finished off with 220. Then same as the epoxy for the primer/filler and paint coats.

Do yourself a favor and get a drafting brush. Use the drafting brush to smoothe out the glass cloth. It has just about the right "drag" to grip the cloth and move it a bit and it also develops a slight static change that heelps the cloth cling to the surface when you're laying on the resin.

Also, when you apply the resin, don't forget to squeegy off the excess and blot up any stubborn wet spots along the edges.

Dave Platt's Videos demonstrate this and it works quite well.
Old 02-11-2008, 06:44 PM
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Default RE: Prep for glass

Hi djr,

I just go down to 220.

Here is a basic "How To" for glassing with waterbase polyurethane (I use MinWax Polycrylic ). I have found that glassing with polyurethane has about 1/2 the weight of a resin job, but only about 60% of the strength/hardness (strength should be built into the airframe). I like the poly method because it is easier to sand, has NO FUMES and is a soap and water clean up. The steps outlined below have been tested and work with no problems (or warping on properly supported sheeting ) down to 1/16". I have not glassed over anything thinner.

Glassing with Water Base Polyurethane

1. Sand the model with 220 grit and remove any highs/lows you may find.
Fill as needed with lightweight filler and sand.

2. Give the wood 1 thin - medium coat of wood sealer. I use the
commercial stuff. What this does is twofold - it stops the balsa
from soaking up too much of the poly and at the same time protects
the balsa from the water in the polyurethane.

3. When dry, sand lightly with 220 to remove the "fuzzies".

4. Some people say to apply a light coat of 3M #77 adhesive to the
wood at this point. I have found that this is more trouble than it
is worth since if you have a wrinkle, the complete piece of fiberglass
has to be removed to straighten it out.

I prefer to lay the fiberglass cloth on the part/area to be covered and
smooth it out using a SOFT brush. Brush FROM THE CENTER TOWARDS THE
EDGES. The static electricity usually holds it in place. I normally
use 1/2 oz (.5 oz) or 3/4 oz (.75 oz) fiberglass cloth.

5. Using WATER BASE polyurethane and a FOAM brush, start at the center
and brush towards the edges of the fiberglass. All you want to do is
stick the fiberglass to the balsa, so excessive amounts are not needed.
Any additional pieces of fiberglass should overlap each other about
1/2 - 3/4 inches.

Let this dry. DO NOT SAND !!

Brush on another coat of polyurethane. This coat can be a little heavier.

Let this coat dry. DO NOT SAND !!

6. Mix up some polyurethane and microballoons. I use 1 part
microballoons to about 5 parts polyurethane. This will be on the thick

Brush on a medium coat of this mixture and let dry.

7. Wet sand this with 220 or 320 grit paper. BE CAREFUL, AS YOU CAN

8. Check the fiberglass carefully to insure the weave is filled. If
the weave is not completely filled, repeat step 6 and 7.

9. SPRAY a coat of water base polyurethane on the plane and let dry.

10. Spray a THIN coat of primer on the plane. When this is COMPLETELY
dry, block wet sand with 220 or 320 grit as much of the plane as
possible to highlight any highs/lows you may have missed during your
sanding/prep. The areas that can not be block sanded, CAREFULLY sand
by hand. (TIP: CA some of the wife's/girlfriends fingernail file boards
together. Wrap the sandpaper around them so you can block sand in tight
areas. The CA helps prevent the boards from disintergrating in the water)
Fill any low areas with a lightweight filler and sand when dry.

Apply a THIN coat of polyurethane to these areas.

When the poly is dry, repeat this step until you are satisfied that all
the highs/lows are removed.

11. SPRAY a thin coat of polyurethane on the plane. When dry, spray
the primer.

12. When the primer is dry, you can apply your rivets, panel lines and
other detailing desired.

13. Now you can spray your paint.

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