Drilling holes in Gel coat

[DIW]

Assistance please. What is the best way to prepare a drilled hole in gelcoat to avoid gelcoat cracking when screw inserted.
Ian

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[tcm]

put a cros of masking tape over wherte you want the hole to be to gain purchaew and to protect against any drill bit "walk". If you don't have have a drill with "oversharp" 60 degree angle of tip for plastics instead of normal nearer 90 degrees, use v fine drill as pilot and open with gradually bigger drillbits. Find an old shitey bit of grp to practise first.

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[DIW]

Thanks tcm , any advice on stopping the small shards of grp flaking off when a screw is inserted, do you think a small counter sink would help ?
Ian

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[tcm]

the small shards and chips are cos either the drill was blunt or not correct shape (if that's when it came off) or cos the hole is a bit too small and the self-tapping screw (or normal screw) cracks open the hole instead of squeezes it open. Again, where it matters, try with a slow sharp drill on a test bit of grp, behind a cupboard or under a seat cushion t get the technique right befoere doing one that is "on show".

Depending on what you are fixing, you may find it better to use a larger hole and decent "hollow wall" fixing which "gives" as well as providing better holding: the screw and self-tappers are really designed to grip all the way down their shank in a solid but "giving"material like wood or chipboard, rather than try and hold firm only near their heads in a few mm of grp where the material is thin and less flexible: the screw widens the hole as it is screwed home, but instead of widening the hole and grabbing the screw tight, a small lump of the grp aroudn the hole can be twanged/chipped off, partly cos the hole wasn;t smooth (blunt dril) and/or partly cos the plastic is brittle and partly cos screws/self-tappers are not the correct fastener for thinnish GRP, really, depite their widespread use amongst modern-day shortcutting bodge merchants who make new boats.

Good luck.

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[capsco]

a counter sink, or better still a larger drill, same size as the screw through the gelcoat, resulting in the screw gripping only the grp not the gelcoat will solve the problem, this will pevent the gelcoat being expanded as per TCM explanation

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[fireball]

alternatively - if you can get behind (loan of a small child might be required!) you could place a block of wood behind an oversized hole to screw into, this has the added benefit of spreading any twisting load over a larger area.

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[Evadne]

And a brass cup washer under the screw head to hide all the chipped bits of gelcoat around the hole when you get it wrong.

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[Talbot]

I drill the hole the correct size to grip the screw, then go up a couple of drill sizes to drill just the gel coat (best done with a hand drill) You can do the same with a countersink bit, but it can cut more gelcoat than needed, on the other hand, there is less likelyhood of cutting beyond the gelcoat and weakening the subsequent fixing.

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[gjeffery]

Depends what you are fastening. Drill a clearance hole, using a pilot first. Then countersink, and bed whatever you are fastening on Sikaflex, which fills the countersink, making the hole watertight.



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[chuns]

Here\'s another tip...

If you are bedding the fitting down on a sealant like Sikaflex, or that grey compound (don't know its name) the countersink advocated by many compresses the sealant onto the bolt or self-tapper making a good watertight seal.

BTW what is the name of the grey stuff - it comes on a roll, each layer separated by non-stick paper, and is about 10mm by 2 or 3 mm. You pull off the required amount, and can shape it to suit - rolling it out a bit like plasticene. It sticks like s**t, but it never goes completely hard. Unlike Sika it doesn't go off in the tube.

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[gjgm]

maybe i m missing the point, but it isnt drilling that is the problem, but cracking when you insert the screw? Thats because you havent drilled a correct size hole; the grp underneath is crushed when you insert the screw and the gelcoat is broken free. If its chipping whne you drill, then i can only support comments elsewhere, but always use some mastic/sealant to stop vibration loosening the screw. if you slightly oversize the hole and its not a heavy duty fixing, you can fill the hole in a bit with some marine filler...
If the grp isnt thick enough to support the stress of the fixing, then you will need to take steps as in other posts of course

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[Dave1258]

Re: Here\'s another tip...

Seam Sealer Chuns..is the word(s) your looking for mate.

<hr width=100% size=1> Experience is something you don't get until just after you need it.

 

[robind]

Re: Here\'s another tip...

Firstly drill a suitable sized hole and then countersink it to at least the full depth of the gelcoat thickness .
Rob

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[Stork_III]

re your "Find a shitey bit of GRP to practise on".

LEAVE MY BOAT ALONE!!!!

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[chuns]

Thanks Dave1258...

... obvious innit?

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[Lee_Shaw]

Re: Thanks Dave1258...

If it's a countersunk screw and you don't countersink the hole to accomodate it the gelcoat'll crack for sure.

Drill the hole smaller than the screw gauge (About 1-2mm is fine depending on gauge) which are as follows - 6's are 3.5mm, 8's - 4mm, 10's - 5mm and 12's are 6mm. No cracking. Fanny's your Aunt. Put some sikaflex in the hole if appropriate.

<hr width=100% size=1>Pretend this is a link to a website.