Screws Damage Gel Coat

[GAJ52]

Is there any trick that stops self tapping screws damaging the gel coat on a GRP surface, the few times I have tried attaching something to my boat, the gel coat cracks and chips.

 

[sailorman]

Is there any trick that stops self tapping screws damaging the gel coat on a GRP surface, the few times I have tried attaching something to my boat, the gel coat cracks and chips.

correct sized pilot hole

 

[davidej]

correct sized pilot hole

....is the correct answer

 

[theoldsalt]

Presumable the screw reaches into something behind the gel coat. Therefore drill a clearance hole on the gel coat so the self tapping screw doesn't try to cut into the gel coat. Maybe that is what "sailorman" was getting at.

 

[prv]

As well as the correct size pilot hole, if it's somewhere highly visible with smooth gelcoat then I just touch a rather larger drill bit to the hole for a revolution or two, to form a slight countersink in the gelcoat only.

Pete

 

[maby]

As well as the correct size pilot hole, if it's somewhere highly visible with smooth gelcoat then I just touch a rather larger drill bit to the hole for a revolution or two, to form a slight countersink in the gelcoat only.

Pete

You beat me to it! Assuming that the screw is reasonably long, then drill through the gelcoat completely with a slightly larger drill, apply some Sikaflex or similar and insert the screw.

 

[sailorman]

Presumable the screw reaches into something behind the gel coat. Therefore drill a clearance hole on the gel coat so the self tapping screw doesn't try to cut into the gel coat. Maybe that is what "sailorman" was getting at.

Pilot not clearance

 

[sailorman]

You beat me to it! Assuming that the screw is reasonably long, then drill through the gelcoat completely with a slightly larger drill, apply some Sikaflex or similar and insert the screw.

Then a wood screw would be more appropriate

 

[maby]

Pilot not clearance

Are you sure? In my book, a pilot hole is quite small and serves to centre up future drilling and cutting operations. A clearance hole is slightly larger then the item to be inserted. In this case, theoldsalt, prv and I are all suggesting puncturing the gelcoat with a hole that is slightly larger than the screw shank to stop it cracking the gelcoat - that is a clearance hole in my book.

 

[prv]

Pilot not clearance

Pilot hole in the GRP, clearance hole in the gelcoat.

Since normal drills have a conical end, and I don't want to enlarge the hole in the GRP, the gelcoat bit ends up more like a countersink.

Pete

 

[prv]

Are you sure? In my book, a pilot hole is quite small and serves to centre up future drilling and cutting operations. A clearance hole is slightly larger then the item to be inserted. In this case, theoldsalt, prv and I are all suggesting puncturing the gelcoat with a hole that is slightly larger than the screw shank to stop it cracking the gelcoat - that is a clearance hole in my book.

It's a self-tapper, so there isn't a shank as such. But fractionally larger than the thread major diameter, yes.

Pete

 

[sailorman]

Are you sure? In my book, a pilot hole is quite small and serves to centre up future drilling and cutting operations. A clearance hole is slightly larger then the item to be inserted. In this case, theoldsalt, prv and I are all suggesting puncturing the gelcoat with a hole that is slightly larger than the screw shank to stop it cracking the gelcoat - that is a clearance hole in my book.

with a self tapper it would be the dia of the centre core of the screw leaving just the thread to cut into the grp

 

[neilf39]

As has been said, use a countersink so the brittle gelcoat is removed to just outside the screw thread. I assume you are screwing something down onto the fibreglass as its a self tapper so the screw head is not directly onto the fibreglass. Small amount of mastic to seal it when screwed in.

If you are screwing into something behind the fibreglass then make the hole big enough for the screw to push through as it should screw into the object behind.

 

[GAJ52]

Ok thanks for all the good suggestions.

From what I gather its pilot hole followed by gel coat clearance hole then some Sikaflex to seal the hole.

I'll try this next time - thanks

 

[wiggy]

Countersink.

 

[nigelmercier]

Pilot hole in the GRP, clearance hole in the gelcoat...

Agreed this is the way, I imagine there is a table of drill sizes somewhere.

It's quite tricky to prevent the clearance hole from grabbing if you drill the pilot first, so I'm never sure of the best way to proceed.

 

[xeitosaphil]

Agreed this is the way, I imagine there is a table of drill sizes somewhere.

It's quite tricky to prevent the clearance hole from grabbing if you drill the pilot first, so I'm never sure of the best way to proceed.

Most if not all metal fittings will have been drilled with a clearance hole for the fixing. If you size the clearance hole with a drill same size, position the fitting where you want it, and just touch the F/G gel coat to countersink it, not only have you created a recess for the sealant to seal the fixing, but you have also provided a centre for the pilot drill or clearance drill depending on whether you want to fix to the F/G, or fix through it.

 

[Javelin]

Gelcoat has no give in it whilst the glass laminate does, the screw needs to bite and hold in the laminate and not in the gel so counter sinking removes it and provides a good area for sealing any hole.

My process of fitting just about anything into gelcoat is,
Pilot hole > countersink
Then normally I put the screw through the fitting to be attached and wind some butyl rubber round the thread up close to the base of the fitting.
Then screw down.
Leave for a few hours or over night and then tighten slightly.