krichart

Do I need bottom paint?

12 posts in this topic

I use my Fiberglass boat on a freshwater lake. it is a 19 foot Four Winns runabout. I was once told that if I keep the boat in the water year-round that I need to bottom paint. The reason given was not to prevent fouling but that the gel coat is porous and water will slowly leach into the core material in the hull. Bottom paint - according to my source - will create a water-tight seal.

 

I am skeptical of this advice because I know many people that keep larger craft in the water continuously for years at a time. Can anyone shed some light on the need for bottom paint for this application?

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Gel coat is in fact porous and over time will allow water to penetrate the core.  Applying bottom paint will slow down the process but eventually will allow some water to leach into the core material.  If the hull is made with polyester resin, it is more susceptible to osmosis and blistering.  Vinyl ester resins and epoxy resins are much better at preventing osmosis.  If you are going to keep your boat in the water for long periods, it would be best to have an epoxy barrier coat like Interlux Interprotect 2000E.  You would then topcoat that with a bottom paint to eliminate any slime or growth.

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Hi, This is my first post, hopefully I have got the program right.  I am restoring a 1968 13 foot Whaler.  I have cleaned, de waxed and sanded the bottom.  I have filled the dings and dents with Total Fair and plan next to apply Total Protect.  After washing and sanding the first coat I will glaze with 3M putty.  Then, a couple more coats of Total Protect.  After that, I had hoped to apply a two part polyurethane, but no bottom paint.  The boat will sit in the St Lawrence all Summer.   Finally, my question:  Is this approach OK or must I paint on an anti foul bottom paint?  Thanks.

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If the boat is going to be in the water more than a couple of day, any paint other than a bottom paint will blister and fail.  You will need a bottom paint even though it's in fresh water.  You could use an inexpensive basic bottom paint like JD Underdog.

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Hi,  Thanks.  I appreciate your counsel and will do as you recommend.  Just a couple of final questions: do I apply the JD Underdog over the Total Protect or the two part poly?  And, given the unusual bottom of an old Whaler, where does the bottom paint stop and the top side paint begin?  I marked the wetted area on the bottom.  Thanks again.

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You will apply the bottom paint after your last coat of Total Protect when it is thumb tacky (when it leaves a thumb print in the paint but no paint comes off on your thumb).  Tape off where the wetted water line was and go up about 1 1/2" to 2" and put the tape there.  That will have bottom paint a little above the waterline which will keep the 2 part poly cleaner if it isn't slapped by wave action as much.  I find that it's easier to paint the topside first before the bottom paint.  It sound like you are going to put Total Protect on the whole hull topside and bottom.  Tape off where you will go up to with the bottom paint.  Then apply the polyurethane topside paint.  Remove the tape after you finish that.  Let it cure 7 days, then tape that off ready to apply the last coat of Total Protect and then the bottom paint when the Total Protect is thumb tacky.

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Hi,  Got it.  Last question: since I'll be applying bottom paint, can I substitute Wet Edge Top Side paint for the two part poly without giving up too much?  My assumption is the single part will be easier, less expensive and less toxic, but still provide a nice finish and good protection.  I have had good experience so far with your JD Total Boat products.  Thanks again.

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You can use a single part polyurethane like Wet Edge instead of a two part polyurethane on the topsides.  It is easier to apply and maintain than a two part.  The two parts typically last about 10 years with good care where the single parts will go 5 plus before you need to do a light sanding and another coat or two.

 

If you don't mind the work involved in applying another coat, sometimes it works out better to get a fresh new coat after a few years to cover up all the dings and scratches that happen with use.

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Hi,  I have followed your advice and have had good results.  Thank you.  The Whaler has been sprayed several coats of Wet Edge and is now ready for Underdog.  We sprayed the Wet Edge because the first coat of roll and tip showed too many brush marks.  A $25 dollar Harbor Freight gravity sprayer did a great job.  My question: can we apply the bottom paint now when the boat is upside down even though we'll miss the launch window by six months?  Can I just abrade the bottom paint with a Scotch Brite pad just before the Spring splash to refresh the anti fouling?  Thanks again.

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Underdog is a single season bottom paint and needs to get in the water within the launch window to be effective.  Abrading with a scotch brite just before launching would help but that is usually done on multi-season paints, not single.  Since it is going to stay in the water all summer, I would wait to paint just before launch.

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Hi,  Got it.  If I am OK with some fouling, but still want to protect the bottom from water intrusion, can I paint the Underdog now, knowing it is well before the launch window?  I ask because our other boats are lifted out of the water in the Fall and show very little bottom fouling.  And, we still have a fair amount of work to do once the boat is right side up; so I really hope to not have to flip it again before launch.  Thanks again.  

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When your last coat of Total Protect Barrier coat is thumb tacky apply a coat of Underdog bottom paint.  If the TotalProtect has fully cured, then sand with 80 grit and apply the bottom paint.  If you can wait until just before the boat goes in the water for a second coat that would be the best for antifouling protection.  If not you may experience some growth.

 

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