I spoke to a chemist from Grangers the Company who make Fabsil waterproofing liquid and here is a summary of the chat we had regarding water proofing and using Fabsil.
I was told that before polishing a car you would wash it! I was advised that it was the same with a tent. Wash the area to be treated before attempting to waterproof it. Wash with Tent Clean a Grangers product.
It was explained to me that more expensive tents have better base materials and are unlikely to have the stitching stretch so much that they leave a hole at the stitching point. Cheaper tents when they are strained pull against the stitching and can open a hole through which water runs! Only solution is to seam seal the hole if the tent is not seam sealed already.
I was told that sun damage can damage a tent in a matter of a couple of weeks.
Mildew lives on dirt and any droppings on the surface of a tent. To kill mildew it often happens that the base material becomes damaged also.
Synthetic materials have a UV coating that can be washed away when treating mildew.
Fabsil is not a replacement for material that is damaged. Fabsil works excellently on synthetic tents.
If the tent has been proofed and is new and still leaks then the problem probably is the material.
A mild solution of Milton maybe too strong a cleaner for mildew and may take away the base of UV coating. If this happens Fabsil will be unable to fix this particular problem. Fabsil makes the water bead and on treated material, shaking it will make the water fly off.
If there is an open stitching hole then that needs to be treated with a seam sealer.
Finally O Meara camping have been selling all sorts of tents for more than 50 years. In that time there has never to my memory been a tent that did not leak at all at all even if the leak was a tiny one.
Modern tents can be sold as having hydrostatic heads of 5000mm. These modern tents suffer the same as tents long ago. Condensation and leaking can take place and above all the tents need to be weathered. The stitching used in the 1000s of needle holes is designed to swell when it gets wet. It stays swollen and sometimes it needs to be weathered more than once before becoming waterproof.
Hope that all helps!