If you were going to coat with fiberglass what ply to use?

[brian_nj2006]

I'm planning on coating the exterior of my trailer with fiberglass mat and epoxy resin, but am unsure on the thickness of the plywood to use as a backer.

I plan on using only 6 oz. mat so it is not that thick/heavy, but need the structure to take the trailer on unmaintained roads and trails. Any sugestions on this one?

[mikeschn]

Hey deeds, not words...

That sounds like a question for Steve Fredrick... He's our fiberglass/resin expert... Hmmmm wait a second, so is Andrew! Oh yea, there's a couple others too... I guess we'll just wait for one of them to "chip in". While we are waiting, pickle juice anyone?

Mike...

[Steve Frederick]

I build a sandwich wall 1/4" ply-foam and frame- 1/4" ply. The wall is very strong in itself, the 'glass is a mechanical/moisture proofing element. When I say mechanical. I mean that it keeps the sections of ply (edges) from printing through(where the seams develope a crack) the finish. I use 4-oz plain-weave cloth and laminating epoxy from Raka I've used this formula on four T/D's and two kayaks, with no problems!

[brian_nj2006]

Steve one more ? if I may. Did you have to used thickened epoxy on the seems before glassing it? Being that my trailer is 10' long and 5'6" tall I will have two seems on each side I can get the joints tight, but was worried about them seperating and showing through.

[Juneaudave]

Brian...were you going with a solid wall or a composite wall?

[brian_nj2006]

Wall will be a ?" ply - insulation - 1/4" ply sandwich

[Steve Frederick]

Steve one more ? if I may. Did you have to used thickened epoxy on the seems before glassing it? Being that my trailer is 10' long and 5'6" tall I will have two seems on each side I can get the joints tight, but was worried about them seperating and showing through.

If you are glassing the sides, then painting, just make a schmutz of wood dust, from your sander, and epoxy, and some fumed silica (get it with your epoxy). The schmutz should be about the consistency of creamy peanut-butter. Once the cracks are filled/cured, scrape the surface smooth, and 'glass away!
If you are doing a bright finish, clear wood, try to make the schmutz from dust from sanding a piece of scrap of the same wood as the area you need to fill. Do some test areas first.
When I cut the sections for the sides of my Tears, I use a jig that allows me to keep the seams to a fine line. I just force epoxy into the seam, through the 'glass in one step.

[brian_nj2006]

Thanks! Still learning here but getting there. This is going to be my woods trailer. Plans are in the works for a future project along the more traditional TD design possibly a cabincar. So this one is my practice.

By the way nice work on the tear Steve!

[angib]

Brian,

It's worth doing a double-check on what you mean. Are you proposing to use 2oz chopped strand mat? If I'm remembering my non-metric weights correctly, chopped strand mat is measured in oz per square foot, with usual weights of 1, 1.5 and 2 oz/ft2.

However glass cloth is measured in oz per square yard, so Steve's 4 oz/yd2 is only 0.44 oz/ft2 - you are proposing to put on five times this weight of glass.

In addition chopped strand mat will usually take 2.5 times its own weight in resin to properly wet it out, while glass cloth requires only 1-1.5 times its own weight. So your plan to use 2oz mat means you'll be buying 10 times the amount of epoxy that Steve would for the same area.

It's no problem if that's what you want to do, but if you're choosing mat because it's cheaper, it's likely you'll pay much more than the saving in extra epoxy.

And just in case you are a fiberglass novice, it makes a big difference how the chopped strand mat was made if you're going to use it with epoxy. Powder-bound mat works fine with epoxy but emulsion-bound mat does not, so you need to know that you're buying the powder-bound type. Sadly, there is no way of seeing the difference

Andrew

[brian_nj2006]

Andrew,

I am planning on using mat, due to its availability (and its free) and also using west systems epoxy for bonding the mat and the top coats. got a real good deal on clearence. The added weight should not be a problem due to the vehicles that will be towing it, lowest tow rating is 5500 lb.

As for experience. I have never done anything on this scale with fiberglass before but I have made a few boat repairs both cosmetic and structural in the past and they are all still holding up. I hope I have as much sucess with this one

[doug hodder]

Brian...as a boat builder and a tear builder that uses epoxy...you may find that your good deal on mat is completely lost due to the expense incurred in getting enough epoxy on it...cloth is relatively cheap and if you have to buy at least twice as much resin...you've lost your good deal....I'm with Andrew on this one...just my opinion...Doug

[asianflava]

I'm not positive, but I think I remember that chopped mat provides minimal strength. Not much additional strength over the epoxy alone. It's purpose is to build thickness.

[angib]

I'm not positive, but I think I remember that chopped mat provides minimal strength. Not much additional strength over the epoxy alone. It's purpose is to build thickness.

I think I should speak in defence of chopped strand mat in case nobody else does! It isn't as bad as you're saying and its apparently poor performance is because it ends up with so much more resin than woven or non-woven glass cloths. The extra resin doesn't make it any stronger but increases its thickness. Strength (stress) is measured as load per cross-sectional area, so the thicker material measures as weaker.

Actually 1 ounce of glass per square foot in chopped strand mat is not much weaker than the same weight of glass in cloth, but it will weigh at least twice as much when laminated.

If you're trying to add water protection, not strength, you could argue that chopped strand mat is OK.

But, Brian, the requirement for powder-bound mat remains - even free emulsion-bound mat doesn't work with epoxy!

Andrew