Different Skin Weights

Finishes, paints and coatings

Different Skin Weights

Postby dshawster » Wed Feb 22, 2017 11:45 am

I have searched previous posts and have been unable to find an answer, this topic has come up many many times before I'm sure of it! What are the typical weights of different skin options per sq ft? Aluminum, Fiberglass and Canvas? The aluminum is easy enough to find out with all the information on line, but fiberglass and canvas are a different story. I know it will vary person to person depending on how its applied but maybe someone has calculated a 'typical' weight? Thanks in advance!

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Re: Different Skin Weights

Postby rruff » Wed Feb 22, 2017 12:25 pm

For canvas or fiberglass, multiply the weight of cloth by 1.25 to 1.5 to get the glue/resin weight. Total weight will then be 2.25-2.5x cloth weight. You might need to add a little for soaking into the substrate (plywood? textured foam?) and you will definitely need to add for the final coating.
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Re: Different Skin Weights

Postby dshawster » Wed Feb 22, 2017 12:31 pm

Excellent, thanks for the info!

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Re: Different Skin Weights

Postby gizmotron » Wed Feb 22, 2017 12:48 pm

I'll try some arithmetic to show some weights.

From this:
Cotton Duck Canvas 10 oz / sq. yard -- ( 9 sq ft in one sq yard ) 10oz divided by 9 = 1.1 ounces per sq ft
Cotton Duck Canvas No. 12 - 11.45 oz / sq. yard -- 11.45oz divided by 9 = 1.27 ounces per sq ft
Cotton Duck Canvas No. 10 - 14.73 oz / sq. yard -- 14.73oz divided by 9 = 1.63 ounces per sq ft
Cotton Duck Canvas No. 8 - 18 oz / sq. yard -- 18oz divided by 9 = 2 ounces per sq ft
Cotton Duck Canvas No. 4 - 24.25 oz/ sq. yard -- 24.25oz divided by 9 = 2.69 ounces per sq ft

The weight per sq ft of Titebond II for two well soaked coats (before and after) is not known by me and can't be found online at Titebond website.
I could guess that it is probably 2 ounces per sq ft. It's probably less.

1/4" plywood ( 32 sq ft ) weighs from 17 to 20 lbs. The Sande plywood at Home depot is lighter. ((20 X 16) / 32) = 10 ounces per sq ft.

P.S. I see rruff just posted the formula for glue weights. Kool

So, 18oz duck canvas, with glue, will be 54 ounces per sq yard ( 6 ounces per sq ft )
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Re: Different Skin Weights

Postby gizmotron » Wed Feb 22, 2017 1:14 pm

OK, I'm about to consider something else. It looks like that 1/4" Sande plywood comes in at 8.5 ounces per sq ft. So now I need to find a really great wood protection for the plywood. I just spent 2.5 years in a covered wagon made with 1/4" Sande plywood over 2" Owens Corning insulation. That was protected with Behr Waterproofing Wood-Protector. I would add a new coat in about 30 minutes using a paint roller about every six months. The only damage came from not covering the edges of the plywood.

So a really good marine paint with metal edging looks really easy. I'm guessing that I will be adding about 50 lbs in all to it. But it will be bomb proof. This solves the hold down problem. It also adds to the rigidity of my cantilever issue.

Now I'm looking for paint coatings or clear stain coatings over plywood.
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Re: Different Skin Weights

Postby QueticoBill » Wed Feb 22, 2017 9:00 pm

I think I'm looking at similar issue, wanting to just paint ply. Covering edge in metal or plastic just seems too likely to cause moisture to collect so am thinking about treating edges with epoxy before painting.
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Re: Different Skin Weights

Postby gizmotron » Wed Feb 22, 2017 9:47 pm

I'll use PMF under the trim and silicon glue in the screw holes. I used to put a drop or two of epoxy in the screw holes on my skies. I agree that those screws need something to prevent leaks.
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Re: Different Skin Weights

Postby KCStudly » Fri Feb 24, 2017 9:23 am

My testing (linky) showed that the weight of 10oz canvas glued on is surprisingly nearly identical to the weight of 2 plies of 6oz FG/epoxy laid up (prior to filling weave or adding any top coats); about 1.5oz/ft^2.
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Re: Different Skin Weights

Postby rruff » Fri Feb 24, 2017 11:24 am

KCStudly wrote:My testing (linky) showed that the weight of 10oz canvas glued on is surprisingly nearly identical to the weight of 2 plies of 6oz FG/epoxy laid up (prior to filling weave or adding any top coats); about 1.5oz/ft^2.


Thanks, hadn't seen that before.

At the bottom of page 227 you say the weight of the FG is about 3 oz/sq ft though, not 1.5 oz. 12 oz of FG cloth alone would be 1.33 oz/sq ft and if resin is 1.5x, then it comes to 3.33 oz/sq ft or .21 lb/sq ft. I got about .15 lb/sq ft for 10 oz canvas and TB2 so it's close. Straight TB2 and I added a coat on top to fill the weave pretty well.

It can vary though depending on your substrate. I was always over carpet-seam-rollered foam which takes some resin to fill for the first layer. I'm using epoxy and silica thickener now to fill the texture first and that seems to work well, and saves some weight and money too.
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Re: Different Skin Weights

Postby KCStudly » Fri Feb 24, 2017 2:29 pm

Well that's embarrassing for a second time. Deja vu can be a PITA. :lol:

The key thing that stuck with me, despite my math errors, was that the two methods using these common weights of material and schedules are substantially similar in weight.

(... although the total finished weight of the FG will probable end up a tad heavier due to having more weave to fill.)
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Re: Different Skin Weights

Postby Esteban » Fri Feb 24, 2017 8:02 pm

If you used epoxy and 4oz. fiberglass cloth at an ideal 1:1 ratio of epoxy to cloth weight it would weigh about 8 ounces per square yard. That's .88 ounces per square foot. If you used 50% more epoxy it would weigh about 1.33 ounces per square foot.

Six ounce fiberglass cloth and epoxy at a 1:1 ratio would weigh about 1.33 ounces per square foot. At a 1.5:1 epoxy to fiberglass weight ratio it would weigh about 1.66 ounces per square foot.

Imagine a 4' x 8' teardrop with about 125 square feet of surface area for the sides, doors, roof and hatch. 125 x.88 = 110 ounces. Divide that by 16 to get pounds. 110/16 = 6.875. The fiberglass skin would weigh about 6.875 pounds.

Or imagine a bigger 5' x 10' teardrop with about 160 square feet of surface area for the sides, doors, roof and hatch skinned with heavier 6 oz. fiberglass at a 1.5:1 epoxy to fiberglass cloth weight ratio. 160 x 1.66 = 256.6 ounces. Divide by 16 to get pounds. 256.6/16 = 16.6. Its fiberglass skin would weigh about 16.6 pounds.

In both of the above scenarios a fiberglass skin would weigh considerably less that skinning with aluminum.
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Re: Different Skin Weights

Postby KCStudly » Fri Feb 24, 2017 9:17 pm

TPCE is about 9'-8" long x 64 inch wide x 51 inches high at the peak with about 3-4 inches wrapped under the floor, plus a bump out for the front locker.

Rough layup with 2 plies of 6oz I figured was about 38+ lbs. Lighter glass or fewer plies did not give me the damage resistance that I was looking for.

PMF had more flex, and there is the self healing asset, so I would not dismiss those virtues, but to go to the trouble of glassing, I would not bother with lighter cloth; at least not for my application and design criteria. If you want to build an ultra light or motorcycle trailer, then maybe one ply of 4oz cloth would be an asset.

Pays yer money, takes yer choice. :lol:
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