I have a large marque type tent (aprox 20 * 15 feet) which needs waterproofing. any advise on product & method would be great please.
The best suggestion is to go to a camping store and buy commercial seam sealer and waterproofing supplies. A lot can depend on the type of fabric and where it is leaking.
Most likely your tent is leaking around the seams. This is because there are tiny pin holes in and around the seam created by the sowing process. If this is the case, seam sealer alone will probably do the trick. Seam sealer can come in two types, seam tape or liquid seam sealer. A urethane-based seam sealer is better because it is more flexable in cold tempertures and creates a better bond with the tent fabric. A disadvantge is that it is messier to use than tape.
If the fabric itself is leaking you can get a silicone-based water repellent treatment for all kinds of tent and tarp fabrics. Most pretreated tents come from the factory with the coating on the inside. If you touch the inside of the tent with your fingers, the oils on your fingers can react with the treatment causing it to become less effective. Spraying it on the outside can help by preventing the material from absorbing water, causing it to be lighter when carrying a wet tent. Products with UV treatment can slow fading and reduce fabric degradation from harmful UV rays.
Just make sure that you get the right product for the fabric of your tent. People at the camp store can help you pick the correct product and instruct you on how to apply it.
If you wanted to make your own you just need a substance that doesn't mix with water. Some items that you could use includes...
Lanolin - boil some sheep's wool and scrape the oil of the top. Do you have any sheep? This is flammable.
Coal Tar - Heat powered coal over a fire in a metal container and collect the liquid evaporating from the heated coal with a pipe leading from the container into a series of bottles. This is creosote and is used to waterproof railroad ties. This is very flammable.
Beeswax - You can buy this or raise bees. This is quite flammable.
Flax - Can be taken from the fleshy flax leaves, just underneath the tough skin. Not real flamable but will burn.
Making your own will work, but why?
I hope that answers your question. If you need further information please feel free to contact me here or at firstname.lastname@example.org