Angle grinder disc for GRP ?

[VicS]

I am going to have to make good the hole where the old ( Ball Head) toilet outlet was so will be buying a cheap angle grinder to chamfer back the GRP around the hole before patching it up. (I'll be using epoxy resin and the appropriate glass fabric).

The question is what type of disc should I get for the grinder. I have what I guess is a standard "metal" disc from a defunct grinder. Will that do or should I get something more suitable for GRP, if so what?

 

[rogerthebodger]

Vic

I use a rubber/plastic backing pad with resin sanding disks on GRP . You can also get sanding disks similar to flat flapwheels but I find they tend to clog with GRP.

 

[Lakesailor]

I used an angle grinder to cut up the grp Micro racer.
I tried both metal and stone discs. The stone one wore away more quickly than the grp.
Metal did the trick. Make sure to use a grinding disc and not a cutting one. It's tricky to get a smooth edge with a cutting disc and it can break if you use it sideways. It's best if you have an already worn grinding disc as it'll have radiused edges.

 

[Rigger Mortice]

FWIW I wouldn't bother to go to the expense of buying an angle grinder just for that job. A couple of coarse files would be fine.

However, if you are looking for an excuse to justify getting an angle grinder , I would be using sanding discs. It will take a few seconds to do it. Watch the dust; there will be a lot.

 

[PuffTheMagicDragon]

Use a disc that is specifically for GRP or ceramics. They are made of shiny metal (therefore do not shatter) and have a band of some 10mm width all round the outer rim on both sides that is embedded with artificial diamond dust. Used on edge it will cut through GRP as if it's going through cheese. Used on the side it will grind away your chamfer without clogging. They are usually around 2mm thick.

Something like this:

 

[pagoda]

I am going to have to make good the hole where the old ( Ball Head) toilet outlet was so will be buying a cheap angle grinder to chamfer back the GRP around the hole before patching it up. (I'll be using epoxy resin and the appropriate glass fabric).

The question is what type of disc should I get for the grinder. I have what I guess is a standard "metal" disc from a defunct grinder. Will that do or should I get something more suitable for GRP, if so what?

Make sure you mask off the heads door while you are doing this - or the dust will get everywhere.

Also.... wear the right protective gear. Glass fibres as fine dust are very BAD for your lungs... DECENT DUST FILTER reqd.. Ditto eye protection- anything slightly bigger than dust coming off the edge of a grinder could easily blind you. Old coveralls & gloves??
People will scoff about safety gear..but eyes and lungs are pretty essential.

Graeme

 

[electrosys]

I wouldn't use a disk for chamfering grp - I'd use a 'barbaric' wire brush instead (graphic attached). All of £1.50 from a 'pound shop'.
With a low level of attack and almost no pressure, this can be used to quickly remove paint and gelcoat - with a steeper angle and a tad more pressure it will feather grp ready to take a patch - and will never clog.

Have been using this technique/ method for many years.

 

[fishermantwo]

Buy two Aldi 125mm grinders, keep one as a spare. After some horrendous abuse first one may give up. Always keep a new one spare. Rubber pad and 36 grit sanding disc. If you use a wire brush head, wear good protective clothing, those bits of wire in bed themselves in all parts of your body.

 

[VicS]

Thanks for the suggestions

Will try the wire brush I think

Will put a new pair of goggles on the shopping list (I normally wear safety spectacles)

I was wondering about this disc. It says for fibreglass.

Sealing the heads door would be a good idea .... if there was one

 

[TimBennet]

I use a rubber/plastic backing pad with resin sanding disks

This was the best advice. You need a little bit of give. You can get disks in all sorts of 'grit' down to about 24, if I remember right. But beware, these really do 'rip through it'. The angle grinder and these pads are to GRP what a chainsaw is to wood: some people can do intricate carving, but most people just make firewood and saw dust.

It's probably best to stick to 80 grit and take care. Clean off the antifouling with paint stripper before you start (clogs the disk and makes even more toxic dust), mark out the edges of your chamfer with a felt pen and stop regularly to see how you're doing. Leave about a millimetre thickness of original glass at the inside edge of the hole.

To stop dust (and there will be a lot) from entering the boat, firmly and completely tape a thin piece of 3mm ply (200x200mm) over the inside of the hole. The curvature of the hull will keep it sufficiently proud. When you have finished chamfering the outside, remove the ply and simply clean up the glass on the inside. Just abrade it slightly. Then put a small plastic covered pad over the inside of the hole, layout a full thickness of glass and epoxy on the outside starting with the full size glass pieces. When kicked, remove the pad from the inside and finished with one full weight layer.

 

[VicS]

Quote:
I use a rubber/plastic backing pad with resin sanding disks
This was the best advice. You need a little bit of give. You can get disks in all sorts of 'grit' down to about 24, if I remember right. But beware, these really do 'rip through it'. The angle grinder and these pads are to GRP what a chainsaw is to wood: some people can do intricate carving, but most people just make firewood and saw dust.

It's probably best to stick to 80 grit and take care. Clean off the antifouling with paint stripper before you start (clogs the disk and makes even more toxic dust), mark out the edges of your chamfer with a felt pen and stop regularly to see how you're doing. Leave about a millimetre thickness of original glass at the inside edge of the hole.

To stop dust (and there will be a lot) from entering the boat, firmly and completely tape a thin piece of 3mm ply (200x200mm) over the inside of the hole. The curvature of the hull will keep it sufficiently proud. When you have finished chamfering the outside, remove the ply and simply clean up the glass on the inside. Just abrade it slightly. Then put a small plastic covered pad over the inside of the hole, layout a full thickness of glass and epoxy on the outside starting with the full size glass pieces. When kicked, remove the pad from the inside and finished with one full weight layer.

Thanks.
It is going to be tricky because the hole is on the centre line, well forward where the hull is still a broad flat V shape.

It has been suggested that I should patch it from the inside but I'm not too happy about that although it would be a lot easier.

 

[Lakesailor]

There is no "correct" answer. I used what I had to hand (erm, several times) and have found angle grinders are capable of very fine scarfs, giving good area adhesion for glassing.

But whatever you find works.

 

[GrahamM376]

Sealing the heads door would be a good idea .... if there was one

If just working from one side, I usually tape the mouth of a plastic bag around the opening on the other side, saves a lot of cleaning afterwards and doesn't get in the way. Just make sure it's not child friendly, with holes punched in it!

 

[William_H]

Angle grider

Yes Vic get an angle grinder. Look for one in a kit with the rubber backing pad for sand paper type discs. I find I use these much more than the cutting grinding disc.
The angle grinder is the new age man's sword. Have fun brandishing it about.... olewill

 

[electrosys]

Well - I've slept on this - and have come to the conclusion that if I were glassing-in just one hole, then I wouldn't go to the expense of a power tool at all ...

Firstly I'd acquire some decent coarse 'glass' (aluminium oxide) paper - either some of the stuff that comes on a roll for power sanders, or (preferably) a fabric belt from a belt sander. The reason being that the abrasive on either of these is well bonded to the support media, and lasts for ages.

Then I'd source a half-round file - or if you can't find one, then split a length of broom handle lengthways to give you roughly the same profile.

Then simply wrap the abrasive-paper around the half-round, and use it as a rasp. With care (and assuming access) you should be able to put a bevel on both sides of the hole. Not too much dust that way, as a bonus.

Then patch as usual. Myself, I'd use a thin ply, epoxy-primed, and pre-glassed backing-plate, but there are many variations on how to proceed with the glassing itself - much depends on what kind of access you have and and what kind of cosmetic finish would be acceptable.

 

[KristiferColumbus]

I am going to have to make good the hole where the old ( Ball Head) toilet outlet was so will be buying a cheap angle grinder to chamfer back the GRP around the hole before patching it up. (I'll be using epoxy resin and the appropriate glass fabric).

The question is what type of disc should I get for the grinder. I have what I guess is a standard "metal" disc from a defunct grinder. Will that do or should I get something more suitable for GRP, if so what?

Don't waste your time & money on a disc grinder Vic.Buy yourself a cheap file from a place like the pound shop.(I bought a perfectly suitable half round type rasp from a place like that & it would do a cracking job). Hope this helps.
Spend the remainder on buying yourself a spare thermostat if you must

 

[Lakesailor]

He wants an angle grinder though

Don't forget. They are the ultimate spanner for rusty nuts.

 

[oldsaltoz]

I don't know anyone who owns a boat that dose not own an angle grinder or two.

Don't faff around with files and paper, get yerself a grinder with 40 grit concrete grinding disk and have the job finished on minutes with a nice rough finish ready for glassing.

Good luck.

 

[PuffTheMagicDragon]

I was wondering about this disc. It says for fibreglass.

Very similar to the type that I had suggested except that yours would be more useful for grinding surfaces. For chamfering the edges of holes I would suggest that you will only be using the outer parts of the disc though holding the grinder at an angle to the work.

 

[Lakesailor]

Let's not get hung up on details. There are many ways to skin a cat. All he wants is a rough sloping edge to glass to.