Jerry, if you are going to use wedges to displace volume in the pourable PU, make sure they are secure or they'll float out. 

Rubber O rings of precisely the right size can be made using rubber cord cut to length. Gluing the ends together is not necessary, as you're going to squeeze it up. I got mine from Polymax - their 30mm silicone cord has a hardness of 60 Shore, which I think would stiffen up on being squeezed. Anything harder probably wouldn't compress easily enough. I intend to form 2 rings sandwiched between ply discs drawn together with 8 bolts. The O rings need to be contained within ply rings, leaving a gap around the mast equal to the thickness of the O rings. I reckon that this would allow the rings to be compressed by 6mm vertically. 

I don't claim any expertise, so I'm probably not the right person to provide advice. I got the idea from a visit aboard Ivory Gull, a Maurice Griffiths Lone Gull, fitted out and rigged by her original owner, David Tyler. Her mast partners are constructed in this way - the mainmast is carbon fibre and doesn't need a mast coat to keep the rain out. The foremast is alloy and does need a coat, to prevent water lodging on the O rings and causing corrosion.

On Arcadian I used a flat rubber sheet about 6 mm thick and about the density of car tire rubber. This was wrapped around the mast and joined and fixed in place by a piece of vinyl tape used to join damp proof membranes under concrete slabs, and was the depth of the mast collar. I then blocked the base of the gap with a short piece of rope of a diameter slightly larger than the gap and filled above with thickened epoxy resin. 

This has worked very well with no squeaking or groaning.

 If the space between the mast and collar needs to be able to be opened up for a larger section of mast or a base plate, then a second rubber sheet could be taped around the collar and the gap between the two filled with thickened epoxy. The rubber would probably need to be thinner, say 3 mm, so that the total join allowed about the same amount of compression as a single layer of rubber. 

The vinyl tape is easily removed and the join between the mast and rubber collar is not glued so should be easily removable. I waterproofed the collar on Arcadian with two layers of canvas bedded in silicone mastic. Not the prettiest but has proved watertight to date. All materials used cost a total of about $30.00 NZ.

Jerry, I can't see any advantage in using polyurethane rubber for the mast step. Although wedges might be used here, I'm using 30mm silicone rubber O rings compressed between plywood flanges to grip the heel, another Tyler innovation. This might be a bit more complex than wedges, my thinking was that this system would be quieter. Other people say their wedges are silent and junk rigs are noisy anyway, with creaking batten parrels etc. sawing at the mast.

If you are concerned about the expense of the polyurethane jollop, perhaps you can combine it with wooden wedges, not necessarily driven firmly home, just embedded to reduce the volume needed by displacement.

Thanks for the advice. I've just purchased some epoxy resin from MB Fibreglass so I'll know where to go for the urethane if I decide to use it instead of conventional wedges.

Just one thing occurs to me. As my mast is parallel from base to well above deck level I would need to keep the internal diameter of the partners large enough to pass the base of the mast with its bonded on urethane chock through when unstepping/restepping the mast. This means I wouldn't be able to reduce my current partner diameter by much at all, requiring a huge amount of urethane to fill the 25mm all round gap.

As I see it, the only solution to minimizing the gap between mast and partners would be to add an external sleeve to the mast, but I'm not keen on that idea. So perhaps softwood wedges might still be the simplest solution for a parallel mast.

Just one more question, is it necessary or even desirable to plug the base of an aluminium mast. My 222mm dia, 5mm wall thickness, spun taper lighting pole section seems strong enough without an end plug, but what does everyone else do?

Jerry,

If you made the partners for softwood wedges, then I'd use them. The pourable urethane is very useful in cases where the gap is too small for wedges, the taper is uneven, the mast passes through at an angle, etc, but it isn't necessarily better than wedges if you have nice conical hole, concentric with the mast. 

However, if you do use urethane, go for the minimum gap through which you can drop the mast, as there's no benefit in having the urethane too thick. Say a 5mm gap.

The step should be as deep as the partners.