fibreglassing bends

[dweeze]

Can one of you knowledgeable lot tell me why when you lay woven mat (200g) over tight bends or angled areas it tends to lift up over the angle.

Example - I've just built a new hatch cover using 3 gradiated layers of ply. When laying the mat down the process I'm using is; wet out the whole area, leave to tack, lay teh first layer of mat and seat it down with a roller. At this point it sits really well on the flat areas but tends to lift up over the gradiated areas even when pushed down.

Any thoughts or solutions?

Thanks - Tim

 

[Avocet]

Glass is surprisingly springy stuff and the fibres hate going round sharp bends. It's pretty normal for this to happen. Ideally, try to avoid sharb bends but if its impossible, you could try a layer of glass "tissue" over the whole lot (while its still wet). The tissue is made up of much thinner fibres so they are much easier to persuade round tight corners and once soaked with resin, it tends to hold the chopped strand mat underneath it down. Another trick I've seen used (but I don't know if I'd want to use it on anything structural!) is to mix a bit of gelcoat resin into the final layer. It's more viscous so that also helps keep the errant fibres under control!

 

[ghost]

Yo,
as avocet say's, tissue.
Or impatiently, get some neat scrap wood, totally cover with cellotape, place the celloscrap tight onto the work and put slight weight on it. (gaffa tape the wood on? leave on floor under housebrick ?) but for a really neat finish you ought to use fine tissue. A wise old git once instilled in me, if it looks alright- stop playing with it, wait until its gone off before you paw at it.

 

[snowleopard]

You say 'woven mat'. Do you mean Chopped Strand Mat or Woven Roving?

If it's CSM the way to get it to lie flat is to take a thickish sheet of polythene and press it down into the laminate while it's wet. While pressing it down you can squeegee out excess resin.

If it's woven roving or other cloths, make sure the strands cross the angle at 45°, not at 90°. That means that individual strands have to turn through a shallower angle. (Apply the cloth diagonally, not square).

 

[oldsaltoz]

G'day dweeze,

Forcing the glass to form a sharp right-angle bend can result in a weak spot, squeezing it onto an external corner may reduce the resin to cloth ratio; if strength is not a factor here then no problem. Tissue will go around a corner but not provide a lot of strength and may require several layers.

You only need a very small radius to get glass around a corner, about the same as a small pencil is enough; if the material you are glassing over can to be sanded or ground to the required radius, you can apply some Micro-fibres to the face and sand the edge to provide a neat finish.

Chopped strand mat can be pulled apart like opening pages in a book to get thinner layers, but it takes a lot more time as each layer has to cure before building up, or it will react the same as a normal thickness sheet.

Don't over roll the material, if you have light spots after no more than 6 passes with a roller, you are not applying enough Poly resin.

I hope this helps

Avagoodweekend......