The area the hardware
goes onto should be finish painted and flat. After the fastener holes are drilled they should then be touched up with an epoxy primer/undercoat (assuming you are using an epoxy based system for priming/undercoating) - if a new boat the paint manufacturer will not have specified a zinc based primer so don't use one of those.
The deck hardware
should be bedded over its whole base surface on a marine
polyurethane type sealant such as Sikaflex 291 (a google
will take you to the Sika site and the data sheets). Sealant should also be sufficient to squeeze down into and seal the fastener holes as well - important with steel as the fasteners will have likely damaged the touch up paint you will have used beforehand - and up into the fastener hole in the hardware to under the fastener's head
so the fastener is sealed right through.
Tighten the fasteners up just enough to squeeze out excess sealant then clean up around the hardware. The polyurethane sealants are messy stuff - I find that a rubber coving tool as is sold for coving bathroom sealants when applied in corners is great for this job. Then wipe around with a rag wetted with methylated spirits (denatured alcohol in USA) which will remove uncured Sikaflex 291 but not too much alcohol else it will affect the cure of the sealant. Use the same to clean your hands which will be sticky by now
After the sealant has cured, nip the fasteners up tight. If you have any sealant mess that you didn't clean up before you will have to mechanically remove the bulk of it now it is set but again alcohol will remove the final traces after doing that, if necessary.
For winches mounted on steel coamings, deck, etc makes sure that you don't seal up any drain holes there may be in the winch
and that the sealant does not get up into the body of the winch through its base fastener holes to affect the operation of the gears, etc.
For things like turning blocks for sheets
, etc that may need to be at an angle to the deck to give a correct lead for the sheet or whatever then a teak
block can be shaped to go between the block and the deck with sealant top and bottom, but I recommend the use of as little wood as possible.
For large based items such as anchor
windlasses I would avoid using a wooden block to take out any camber in the deck to give it a flat base to sit on. The best way for sure on a steel boat is to make a low dam on the deck the same shape as the windlass
base but a little bigger all around. Fill that with epoxy thickened with fibres (you will find instructions for this on the West site I think) so that you end up with a flat surface for the windlass
base to sit on with enough thickness at the crown of the deck so that it has some integrity (say 1/4 inch or so). When the base is cured cove around the outside with epoxy thickened with microballons. Then fit the windlass with sealant, etc as described above.
If the windlass motor
DC supply is not isolated completely from the windlass (I think most are but pays to check) then you need to consider if you need to isolate the fastenings from the steel deck with insulating bushes or not, that depending on how you have done your electrics. The general recommendation is to insulate anyway as the chain is electrically connected to it and the zinc on the chain thus may act as an anode to noble metals underwater on your boat. But if your boat is well painted underwater with an epoxy system I have not found it any problem and our own is not so insulated (we only have one anode that on one side of the fin keel
and that lasts a couple of years so we have a galvanically inert steel boat).
While on windlasses, if it has an integral chain pipe as many vertical ones do, or when fitting a stand alone chain pipe it pays to weld a short piece of stainless steel pipe under and up through the thickness of the deck (welded above and below to seal) so that as the chain goes down it does not whip against the edge of the hole in the steel deck which will create a big rust problem but rather whip against the ss pipe. If the windlass is of the type with an integral chain pipe and is mounted on an epoxy base then the ss pipe can preferably come up through the deck sufficient for fillet weld around the top, that all then buried in the epoxy base.
Hope that is of some help.
EDIT: I note the mention of backing plates
above. These should not be necessary on a well constructed steel boat and will introduce problems of their own, for example with respect to insulating over them and also a hidden area which condensation
will wick under (you will get condensation
of the fasteners). In our own case our deck is a mixture of 4mm and 3mm plate (3mm being used as much as possible to keep weight down). The big sheet winches for 500 sq.ft. genoa
are mounted on the cockpit
coamings pressed out of 3mm steel and these are no problem at all (but stiffness is given by the box shape of the coaming). The windlass is on 4mm deck with no doubling or stiffening whatsoever, but mounted quite close to a deck beam.
If a doubler is needed then I would suggest a plate welded all round but would actually lead you to just stiffening the deck underneath with a bar welded on edge and which only needs to be 3/4 inch or so high.
In the case of our bollard made from ss pipe for taking the anchor load when anchored there is a ss doubling plate welded on top of the deck of sufficient diameter only to stop the chain damaging the deck and under the deck just one bar about 1 inch x 1/8 inch welded on edge aligned fore and aft. None of this has ever been a problem even though we operate in a boisterous area.