+1 on the Fabsil. used it on a clapped-out British DPM smock and it made it nicely water resistant. certainly not water proof, but it gave the old thing a good second life insofar as my needs go.
i've since been in touch with the Granger folks -- makers of Fabsil -- and they told me that i should have used their Fabsil Gold which is apparently formulated for garments whereas the vanilla Fabsil is for tarps and tents and suchlike.
I make my own cotton smocks and either use Fabsil or Nikwax Rug Proof (I need air curing as I do not have a tumble drier...)
Both make a big difference to water resistance without adversely affecting drying times.
I was caught in a 20min snow shower today in one of my Rug proofed smocks today and stayed bone dry.
Well I waxed mine with the home made 'Greenland' wax, to report it works and works well.
The smock can be kind of stiff when it's just been waxed and when it's cold, but wearing it soon eases that up and if it's got any 'decoration' on the smock like say camouflage well the wax does tend to make the colours darker.
The first I tested it to be pleased with the waxing was whilst high level walking in a rainstorm of which was coming in sideways, to find no wind nor damp came through to consider the wax not only stops water but makes the garment more windproof.
Thanks to public interest I have prepared a facsimile edition of my long out of print "A basic Guide to Making your own Fleece and pile Fibre Clothing " book to go along with the facsimile edition of my Outdoor Activities clothing book Unfortunately the original work was lost when the printer...
My long awaited booklet "A Basic Guide to making your own, Budget, Bushcrafting Equipment" is now available. 14 fully illustrated, step by step projects (using the simplest of tools) plus detailed instructions on seaming etc. The projects cover - Drybag, stuffsack,Poncho/basha, Giant Basha...
My apologies to the OP for the hijack
I have also made my own "Greenland Wax" 50/50 parafin wax and bees wax and used that to good effect but find it makes things a bit stiff if you over do it - it is difficult to over do Fabsil or Nikwax wash in