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Bertram31.com General Bulletin Board
Re: cap't patrick level question
Posted By: Capt Patrick McCrary In Response To: cap't patrick level question (Matt)
Date: Monday, 9 May 2005, at 8:51 p.m.
Matt,
I presume your using polyester.
First, do yourself a favor and use gelcoat... By the very nature of fiberglass, you'll have very small bubbles that will equate to pin holes in the surface of your part. You'll spend more time and energy chasing porosity in prep for paint than you will laying a 15 to 20 mils of gelcoat.
The gelcoat will give you a nice porosity free base for your painting. Catalyze your gelcoat at 4% mekp
Next, you should be using layup resin rather than laminating resin. Laminating resin is very thick, used for mixing with fillers and then laminating panels or other objects to one another. You'll have a much harder time wetting out your glass with laminating resin.
When doing composite structures you want the thicker skin on the back side of the loaded surface. For a deck panel that's the underside.
24 oz roving is way too heavy for your hatch cover, will eat up a lot of resin, and print through to your finished side. The layup schedule I'd use for the outer skin is:
Gelcoat 15 - 20 mils wet. Allow to cure until you can't dig a fingernail into it.
Mix enough resin to brush on a very thin but complete coat over the gelcoat. Catalyze to not more than 2%. Allow to cure until firm but lightly tacky.
Lay one layer of 1 1/2 oz mat, very wet, no bubbles larger than a fly speck. Allow to fully cure, 24 - 36 hours. Lightly sand the surface with 80 grit to get rid of stray fibers or radical bumps that will be hard to lay over.
Allowing the skin coat to fully cure will help prevent print through of the next layers. Your skin surface should still be slightly tacky. Re-activate the surface with a wipe of acetone a few minutes before laying the rest, if necessary.
I personally don't like balsa core, especially for decks and other horizontal surfaces. 3/4" H80 Divinycell would be a far better core.
If you're really set on trying vacuum, do several dress rehearsals and even a few test pieces before attacking your actual project. It ain't anywhere near as easy at it may sound... I rarely use the technique. A sheet of plastic sheeting, a piece of flat plywood, and a bunch of cement blocks will do the job nicely.
If you do use vacuum assist, you only want enough vacuum to draw out the air, about 26" Hg. You'll also need peelply & breather cloth to spread the vacuum and soak up the excess resin. Be sure to have a catch chamber to seperate the drawn in resin from the air. Vacuum pumps don't run well with hardened resin in the cylinders...
Remember that once you add the mekp to your resin, the clock is running. Have all of your ducks in a row before declaring "Fire In The Hole". Have your glass & core cut and fitted, all supplies close at hand, and go over your game plan several times before going hot...
The next 3 layers, (2 glass & 1 core), can be laid without curing between layers.
1 layer of 1 1/2 oz mat, followed by a layer of 12 oz 45°/45° biaxle, (far stronger than roving & finer surface stitching). Catalyze at 1 1/2% for a longer working time. Also the heavier the lay up, the greater the exotherm & the faster the resin will kick.
Make your layup on the very wet side so that the core can compress into it and won't rob the glass of resin by wicking it out.
The core thickness will play a big part in how stiff your composite will be. The thicker, the stiffer since you are spreading the two fiberglass skins further apart. It's a tension vs. compression thing. The core should be about 1/4" thinner than your finished panel is to be.
The gelcoat, 2 layers of 1 1/2oz mat, 1 layer of 1208 biaxle will give you an outer skin of about 3/16". The back side layup will be about 1/4", so 3/16" + 2/16" + 12/16" = 1 1/16" total.
Allow at least 3/8" of space all the way around your core. Fill that space with thickened resin, (mud), so that your back layups have a good grip at the edges.
You can use your 24 oz roving on the back side, but the biaxle would still be a far better choice.
If using roving, apply it first over the core, then 1 layer of 1 1/2 oz mat.
If using the 1208 biax, you'll need no mat, just 3 layers of 1208. This will also take less resin to wet out properly. (Biaxle will always yield a better glass to resin ratio than mat or 0°/90° roving)
In either case, roll the back side well and remove excess resin. You only want it wet enough to fill and displace air, but not be drippy fat.
Best regards,
Patrick
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