Drilling GRP

[jsl]

Advice, please: how do I avoid shattering the gel-coat around the hole? Small pilot-hole first? Very low speed?

 

[Ruffles]

Pilot hole followed by countersink to slightly larger than the final hole required.

 

[30boat]

What size of hole is it?For smaller holes I've had good clean results with a flat bit that has to be run at high speed.But you have to keep the drill steady or else the bit will grab and make a real mess of things.Another option is one of those bits (forget the name) that are like ordinary drill bits but with a little point in the middle,The very sharp cutting edges will cut cleanly through the gelcoat.Again it's essential you keep the drill steady without wandering.And use a fast speed without forcing.I would try it first on an old piece of grp and get the hang of it.

 

[savageseadog]

Can't see why it should matter as most fittings overlap the hole. If you are really concerned you could consider a diamond drill from a tile shop. You have to make a guide out of some plastic or ply and (I) use silicon sealant to keep it in place

 

[30boat]

That's also good.

 

[Slinky Spring]

This answer was posted recently to another similar post. Never tried it but it sounds plausable
[ QUOTE ]
Just remember to reverse the drill to drill through the surface of the gelcoat as it prevents star cracks eminating from the drilled hole. Whatever you use to fix the sprayhood it needs to be sturdy as you will often grab on or fall against it. At least I do.
The sprayhood manufacturer will advise on what fittings to use. A good quality marine sealant such as dow corning will make the job waterproof.

[/ QUOTE ]

 

[jsl]

thanks, everyone.

 

[Phoenix of Hamble]

Also, masking tape over the drilling point first..... not only helps to prevent cracking of the gel, but also stops the bit from wandering....

 

[pvb]

Another point - if you're going to use a self-tapping screw, after you've drilled the hole change the drill bit for one about 1mm larger and just drill the gelcoat layer out. Otherwise, gelcoat often cracks when a self-tapper is inserted.

 

[rogerthebodger]

A lip and spur wood drill will score the gel coat before the drill starts really cutting and so will prevent the gel coat cracking

more details here

 

[chuzzlewit]

What a fuss : just use a masonry drill.

 

[wagenaar]

The type of drill you suggest to use is designed specifically for wood. The little tip centers the drill and the rest follows then automatically.

 

[damo]

There is a spurred bit suitable for other materials. I can't be bothered to google for it at the moment, but the spur looks like the tip of a small twist drill bit. I've got several and they do the job very well.

 

[leadersail]

Cobolt drills are the best for GRP.

 

[Strathglass]

[ QUOTE ]
Cobolt drills are the best for GRP.

[/ QUOTE ]

That's what I use much better and long lasting.

Iain

 

[oldharry]

Use low speed - always. High speed creates heat which will melt the resin and can cause the drill to snatch and break.

 

[P4Paul]

I think you are referring to a sheet metal drill, very good for drilling GRP, but as already indicated, countersink after if using self tappers.

If drilling larger holes, look at using a holesaw, but you need to work out the right speed, as the diameter of the holesaw determines the surface speed of the teeth, Beware that the thickness of the Gelcoat will also affect how clean the cut edge looks from the finished side.

My last advice is be confident when drilling, we used to have more damage done to mouldings by new employees because they were frightened by the thought of drilling into the mouldings often drilling too slowly and allowing the drill to wander around. Find some old GRP and practice if you need to. But don't panic, if you get it wrong its not too hard to repair (matching the Gel colour excluded).