Hi.  I have a really nice boat covers on my boats, both are 9 years old, and water proof. They have been great and never let any water in. I had them custom built and travel with them. Boats also sit outside 24/7/365.

I just noticed that the cover on my bigger boat, a 17'6" V hull, is staring to let water in. It still looks great, and I am not ready to spend $400+ on a new cover. Is there anything I can put on the cover to help re-waterpfoof it? I thought about the spray cans of 3M stuff, but that would take a lot of cans and cost $$, and not sure it would work since its water resistant. I thought about Thompson's water proof that you used for wood, cement, ect. I used it on an old canvass tent years ago and it worked OK. Just not sure if it will help or hurt my cover. Any suggestion would help. Here is a picture of the boat with the cover

Go to any canvas shop. Water sealer will do the trick.

If you got nine years out of your boat covers you're way ahead of the game.  Here in AZ they are good for about 2-3 years tops.  Maybe the UV does a number on them.  

Be sure that you get a product that is designed to be used on the material that your cover is made from.  

Been there, done that.  I agree with George.  Go to a canvas shop with your boat cover.  The shop will recognize the type of material it is made from and will recommend the appropriate sealer.  Sealers are not cheap.  Expect to pay no less than $40 a gallon.  One gallon should easily take care of your 17' boat.  One tip I received from the canvas shop was to pressure-wash the cover and let it dry before applying the sealer.

I used to do canvas work for 8 years. Pending on the material there are certain things that can cause water leakage. If it is an cotton blend more than likely its getting old and needs to be replaced, so trying to use a waterproofer might buy you some time. If it is an acrylic blend most will last many many years if taken care of. Acrylics are all waterproof from the manufactuing process, but what happens is they get dirty and that allows the water to seep through. Just like touching a tent when its raining. So I would clean it with a brush and alot of dish soap and get all the dirt off. If there is alot of mold on it use a preasure washer on it but BE CAREFUL and keep a SAFE distance from the cover. If all the dirt is gone it should once again not allow the water to seep through.

Hope this helps.

Very good suggestion Ballistik!!! Most of us wasn't aware that dirt can seperate the membranes and allow water through. How do you people remember all this stuff :

thanks for all the info. its not a cotton blend. so i will try the cleaning this spring. Its a bit cold here now ( 21 degree) right now. so washing is out of the question right now. the boat is outside, but is covered by another cover right now for winter. My winter tarp doesn't let any water in. Its just way to heavy to use all year. I am a truck driver and used to run steel. have you ever seen a steel tarp? They are very thick and heavy, 14 x 20 tarp is over 75 pounds. This spring when I get it ready I will wash it first and see if it works. I bet it will work cause it is dirty.