Drilling GRP

[jon and michie]

Hi - I am just getting prepared to put on lettering on the grp transom garage door.
1 of the holes in each letter is 12mm
Is it wise to use a step drill bit to build up to the final size without any cracking around the hole or maybe is there a better way?

Jon

 

[Freebee]

A step drill will only work if the grp is a couple off mm thick because of the steps on the drill any thicker and you will be cutting steps in the hole....... if nervous find a bit of scrap grp and practice

 

[Refueler]

I prefer the tapered reamers used by carpenters - far better than stepped .. as you can complete to size YOU require ...

Problem is that GRP with a Gelcoat - the Gelcoat can split / fracture .. especially as it gets older and more 'brittle'. To counter this - then I would drill to a smaller hole ... applying only light pressure to avoid drill gouging out Gel etc. Then tapered reamer to final size ...

Pleanty of Carpenters Tapered Reamers on eBay etc .... the other advantage is they generally have far harder metal and so far after many years - have never had to 'dress off' the cutting edges on mine ... they are as sharp today as they were when new.

 

[vyv_cox]

12 mm hole saws are available. This is by far the least damaging tool for GRP.

 

[Refueler]

12 mm hole saws are available. This is by far the least damaging tool for GRP.

Think I'd still go slightly smaller and then ream out ...

 

[McFrancis]

The technique we used on kit cars was to use a blunt drill bit so that it won't grab the gelcoat, even run the drill bit or holesaw backwards to get through the gelcoat.

 

[AngusMcDoon]

Brad point bit. £3 from Screwfix for 12mm. Score the perimeter with light pressure first to prevent chipping. Less likely to overheat and burn than a hole saw. Clean accurate cut.

 

[wonkywinch]

It was quite amusing watching Nestor using a hole saw with bare hands as his drill chuck wasn't big enough for the pilot drill.

 

[AngusMcDoon]

.

 

[vyv_cox]

Brad point bit. £3 from Screwfix for 12mm. Score the perimeter with light pressure first to prevent chipping. Less likely to overheat and burn than a hole saw. Clean accurate cut.

Seriously? Completely inappropriate tool for the job. I have drilled dozens of holes in GRP, up to 100 mm in diameter, all with hole saws. Never seen any overheating.

 

[Refueler]

Seriously? Completely inappropriate tool for the job. I have drilled dozens of holes in GRP, up to 100 mm in diameter, all with hole saws. Never seen any overheating.

When I cut the hole in hull for speed log on my 1973 Sunrider ... it needed 3 hole saws ... being solid GRP and THICK !! it destroyed them ...

They were not cheap hole saws either.

 

[Minerva]

When I cut the hole in hull for speed log on my 1973 Sunrider ... it needed 3 hole saws ... being solid GRP and THICK !! it destroyed them ...

They were not cheap hole saws either.

I had the similar at the weekend. Drilling out a larger diameter hole for new log in the hull of my moody. Thought “there’s a lot of dust here” before realising it was smoke poring out the back of the makita drill.

Oops!

 

[jon and michie]

Many thanks for the replies - Much appreciated
Jon

 

[Purple Velvet]

Step drill is all I have ever needed. Run it back through from the other side if thicker than one step.

 

[2Tizwoz]

Many thanks for the replies - Much appreciated
Jon

Diamond core drills can be used gently

 

[AngusMcDoon]

.

 

[vyv_cox]

When I cut the hole in hull for speed log on my 1973 Sunrider ... it needed 3 hole saws ... being solid GRP and THICK !! it destroyed them ...

They were not cheap hole saws either.

Yes, powdered glass is pretty abrasive stuff. But the grp should be in pretty good condition afterwards.

 

[Refueler]

Yes, powdered glass is pretty abrasive stuff. But the grp should be in pretty good condition afterwards.

Biggest worry was the outer Gelcoat .... not wanting it to crack or splinter being old ... so I used Hole Saw till nearly through from inside ... then put back one of the worn out so it was slower and less agressive cut.

Other holes I've cut .. stanchion bracing bolts etc .. its always been a case of preventing the Gelcoat from being damaged around the hole ... joys of old boats ! I've tried all sorts of drills .. HSS ... Wood ... Masonry ... and TBH - they all can damage the surrounding Gelcoat if too agressive ... so I started using worn out drills to start the hole .. then HSS to finish.

 

[andsarkit]

Diamond core drills can be used gently

I have some Parkside diamond core drills from Lidl that came with a suction guide to help start the hole in the correct location. I have only used them on tiles but I expect they would work well on GRP.
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