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How to glass

3K views 17 replies 10 participants last post by  Siamese 
#1 ·
This picture is of a piece of construction plywood made into the top piece of the centerboard trunk that also is a support for the mast of a dinghy.
It is for my rainbow dinghy if you remember my prior post.

I have some fiberglass cloth and some epoxy so I figure I might as well cover it.
Covering a flat surface with cloth is easy enough.

This piece has so many corners and curves, any tips on how to cover it smoothly to cut down on the sanding.
 
#2 ·
Coating with epoxy may be a good idea, but I am not sure why you would need to coat an internal part with fiberglass, so long as the boat was designed without it.

If you do want to coat it, the lighter the glass the easier it will conform to complicated curves. So instead of a heavier glass try switching to 3oz or so. If its really weirdly shapped you can go down even further. The lighter the glass the easier it conforms, but the more layers it takes.

If you want to do the best job possible, after all the glass is layer up wet, encase the part in a vaccume bag, and pull down a reasonable vaccume. This will help apply preassure evenly across the part, and press the glass into complicated curves.
 
#3 · (Edited)
the weight of the cloth has less to do with the way it will conform to compound curves then the type of weave, looseness of weave and size of the tow. the standard boat cloth with it's plain weave is the worst. the cloth with the smaller tow numbers will form to a smaller radius. using a loose 4x4 twill or 4 harness satin weave works much easier. also orientate the cloth so the weave is at 45 degrees to the curved surface and it will lay around the curve easily. one of the best cloths for compound curves is #7725. it is a modified 2x2 twill with a very loose weave about 8 oz.
 
#5 ·
No picture that I can see....?? Be sure to radius the corners well (1/2" or more) or the cloth likely won't lay down all the way around.
 
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#8 ·
I wouldn't even try to cover that in glass - you'll never get any fabric to conform. Cover it with epoxy resin instead. If ease of sanding it is a concern, thicken the resin with talc or microballoons. Don't thicken it too much - you want it to be runny enough to still have some flow so it will self level.

Ease all the edges (corners) first or the resin will be too thin on them. Don't sand the edges after coating, just scuff them with Scotchbrite.
 
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#12 ·
I suggest wetting it with pure epoxy first, if you are going to use any additives during your project.

Regards,
Brad
 
#13 ·
The reason I asked is that I've heard that cheap plywood will telegraph through the epoxy unless it has a layer of cloth.
 
#14 ·
It can but it's not like paint - just put on another coat or put some filler in it.
 
#15 ·
Agreed! It probably doesn't require "glass" for protection or structural reasons.

If you do want to involve "glass" in the layer covering an irregular surface there is another option. Round all saw cut edges with a rasp. Fiberglass mat can be teased apart to a very thin "fabric" layer. Very thin is the key. One layer or filaments is very thin. Mat is chopped glass filaments held together with a seizing. When I want some fiberglass to conform to a tight radius, to add a matrix for abrasion or structural effect, I tease apart some mat and "dab" small pieces into / onto the surface with a stiff "white chip" brush. The brush is applying the resin at the same time. Addin a bit of filler to the resin helps. It is tedious but if you add small amounts of mat in several layers you can get to conform to 90 degree corners. Once a layer of this resin rich mat covering is in place a light open weave fabric can be laid on if it is done while the resin is still tacky. Pay attention to the direction of the weave in the fabric and use small pieces added on the bias. You will end up building some thickness so allow for that. Avoiding sanding is the plan. Having everything ready before you start helps, too.

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#16 ·
If you decide to use mat, remember that most of it is held together with a styrene soluble binder. Epoxy will not dissolve it - if you are using epoxy you must get mat that is specified for that purpose - it uses a different binder.
 
#17 ·
It is correct that there are different binders. It really won't make any difference if you are using the very thin layer I am suggesting for this application. It would be an important consideration if you were trying to get a full thickness layer of mat to conform.

Thanks for keeping it accurate JonB.

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#18 ·
You didn't say why you want to glass it. Can we assume it's for rot resistance?

Also, if you're concerned about the appearance (you said you were concerned about "telegraphing" of the cheap plywood through the mat) why not use a piece of mahogany? Prime it and paint it.

Heck of a lot easier and it will absolutely look good when you're done.
 
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