repair; woven roving or chopped strand

[Spyro]

I need to to a repair to a section of laminate under and around a stantion base, can anyone give me advice on whether to use woven roving or chopped strand mat on the deck area. I also intend to srenghthen the area from underneath by spreading the load onto a backing plate before refixing the base would this be my best option?. Any other advice would be appreciated

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[yoda]

I believe that a professional would use the same as the original. Woven has greater strength in the direction of the fibers where chopped mat has strength in all directions. my personnal preference is to use mat laid in different directions as you build up the layers. I find this easier to work and less prone to turning into a mess as you stipple in the resin. A good backing pad under the base is certainly a good idea to spread the load.

Yoda

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[kgi]

Are you cutting out the area completely IE right through from the deck to the interior? or is it a cored deck and you are going to leave the deck laminate in place and work from the inside?, i havent quite got it clear in my head yet.......keith

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[snowleopard]

the main reasons for using woven (or preferably stitched) roving as opposed to chopped strand mat are (a) weight: csm absorbs at least 2.5x its own weight of resin or 4x if you don't roll it out thoroughly whereas rovings only take 1-1.5x, (b) ultimate tensile strength is higher with rovings.

in your case the area is small so weight is a minor consideration. around a stanchion what you need is stiffness which can best be achieved with a thick laminate so the extra bulk of mat will be an advantage.

when using rovings you should in any case start with a layer of mat and ideally alternate mat and rovings to get the best bond.

the most vital thing is to make sure you sand or grind the area to be bonded. new grp will not stick to old if you don't

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[oldsaltoz]

G'day Ian,

I assume this is not a composite / cored hull. Also I assume it's constructed with standard wax resin.

First point is that I would use only epoxy resin, as wax resins don't give a secondary bond, so epoxy can be 40% stronger.

Don't use chopped strand with epoxy resins, the ratio of resin to glass with chopped strand is too high for epoxy, use only cloth that is suited to epoxy resin: you will find they are easy to work with.

Avoid thick lay-up, or a lot of heat will be generated, I don't put more than 4 layers on at any one time.

If you have to let the epoxy set (Go off) and need to apply more, make sure you wash it with a scourer and fresh water before sanding, or you will rub contaminants back into the job, when the water forms no beads (As on wax) it's clean.

To remove old fibreglass prior to re-glassing, a Dremle tool with a large (1 inch) steel ball makes short work of this task.

I hope this helps

Avagoodweekend Old Salt Oz /forums/images/icons/cool.gif

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[snowleopard]

beg to differ...

on the subject of secondary bonding. my boat is built from grp panels joined together (technique pioneered by derek kelsall).

polyester resins exude a fine layer of wax as they set. this is to form an airtight layer as the pure resin is inhibited by air and would otherwise remain tacky.

nothing will bond to the wax, not even epoxy, so you have to remove it by sanding (most people use a 24 grit disc in an angle grinder. for awkward corners a b&d powerfile works well). applying polyester resin to the prepared surface gives a bond 70-80% of the strength of the original material.

provided your bonding area is adequate this is plenty strong enough. an overlap of 100mm around the damaged area would be enough for most purposes. as an extreme case, the 4 lifting eyes on my 6-ton cat are each secured with a bond area of 300x600mm.

here's a useful tip for ensuring a good bond for such a repair (works for polyester or epoxy): after laying up the patch and rolling with a finned metal roller to squeeze out excess resin, put a piece of thick polythene over the wet resin and squeegee out any trapped air. the biggest enemy of a strong bond (apart from the wax) is air gaps.

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[oldsaltoz]

Re: beg to differ...

G'day Snowleopard,

I agree, nothing will stick to wax not even epoxy, I don't think I said it would. However I did say that Epoxy resin should be washed prior to sanding.

Poly on poly provides only a primary bond, and depends on a good key (with no wax), an epoxy resin gives you a secondary (Chemical) bond as well as a key.

By the way, is you Kelsall Cat a Tonga by any chance?

Avagoodweekend Old Salt Oz /forums/images/icons/cool.gif


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[snowleopard]

Kelsall

no, it's much more recent- a suncat 40 mk2

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[Spyro]

Thanks for advice but what about gel coat?

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[oldsaltoz]

G'day Ian,

Gelcoat repair is another subject, well covered a search on this forum will give you all the information.

The trick here is make the gelcoat area to be repaired as small as possible, so don't cut away any material you have to.

Hope this helps

Old Salt Oz.

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