Rubber nuts.......

[RutlandMike]

Anyone tried Masonmate rubber nuts (Rawlplug do the same thing - called rawlnuts) for heavy duty fixing to GRP where you can't get to the back of the bolt for a nut and washer? Makes a waterproof seal in stuff prone to cracking they say. Just wondering how strong they are. See here.

 

[PBooth]

Hi, I have used them to fix roof mounted refrigeration units (about 80 Kg) onto van roofs, using 6 x M8 size. This is through the sheet steel van roof and into 19mm plywood re-enforcing. They give a good solid fixing. On my GRP boat, I have also used 'Rivnuts', which are a metal version available in stainless or aluminium, they are 'set' by a tool similar to a pop rivetter and work fine, very useful if you only have access from the front.

Peter

 

[Poignard]

Are the screws stainless steel? Although if not, it wouldn't be difficult to replace them with s/s ones. Having done that I dont see why they wouldn't be useful for fixing lighweight items to a grp panel. They wouldn't be waterproof because water would creep down the thread of the screw unless it is soft rubber, or you used a sealant.

But personally, I'd rather use nuts and bolts and a backing pad.

 

[RutlandMike]

Thanks guys. I'd much rather use nuts and bolts too but the only way I can access the back is to cut holes through internal GRP mouldings which I'd really rather not do. Good point about the screw threads and water, a dob of silicone should sort that and swapping for ss bolts a must. I'm just paranoid about the hold strength. I'd of thought regrigeration units on the roof of lorries would be a pretty good test. How many did you fix them on with? The GRP I'm going through is 10-15mm thick so not too flimsy but no chance of a back board of any kind.

 

[RutlandMike]

Thanks Peter - sorry, I should have read your reply more carefully - 6 xM8. The rivnuts sound like they're worth a look too - many thanks.

 

[vyv_cox]

I agree that Rivnuts are an excellent product and very well suited to use with metal. However, a word of warning. The action of closing them is very powerful and I have found that they will easily force themselves through thin GRP. Accurate control of the closing tool is needed, particularly the final sheet thickness.

 

[PBooth]

Yes that's right, my rivnut tool has a ratchet action, so I can clinch up the inserts gradually. I slightly countersink the hole to help prevent splitting the GRP. I always use sealant to keep water out

 

[steve_cronin]

The captive nut inside is usually made of brass so just changing the screw for stainless gives you quite a nice fitting which is easily waterproofed.

I've used Rawlnuts for many things over the years and although they're intended for attaching fittings to tiles over wallboard, they would also work well on a thickish GRP panel. They are much less agressive than Rivnuts

Steve Cronin

 

[RutlandMike]

Thanks guys - all much appreciated.