Your personal preference. If there aren’t that many layers of bottom paint, you can sand them off with an 8-inch, dual-action orbital sander and 60-grit sandpaper, but this is hard work and requires careful safety precautions. Overzealous sanding can lead to dings and divots in the gelcoat.
Another option is using a chemical paint stripper like Peel Away or Franmar Soy Strip (PS, October 2011). Chemical paint strippers break down the paint’s adhesive bond on the hull and make it easier to scrape down to clean substrate that can be repainted. This is messy and noxious work, but it’s less back-breaking than sanding. We’ve found that the strippers performance varies by boat (and temperature), so review the articles on strippers to see which one best fits your scenario. And if you’re not happy with one product, you might want to try another before giving up.
Both of these methods will be time-consuming on larger boats. Before tackling this project, check out our October 2011 article, “A Mathematical Decision Maker,” which outlines a formula for determining whether the DIY approach is right for you. On boats in the 40-foot range (or larger), we recommend looking into soda-blasting the hull (PS, October 2011). If you’ve got an old barrier coat that needs renewing, and are still considering a non-blasting regimen, then you’ll first want to read Ralph Naranjo’s account of his barrier coat removal project.
For owners of older boats with unknown, well-adhered coatings, a tie coat can help make sure successive coatings stick. The major bottom paint manufacturers—Interlux, Pettit, and Sea Hawk—all have priming/tie-coat products. One that we’ve recently used with success is Sea Hawks 1277 Barrier Coat Primer, a chlorinated rubber compound. Pettit’s version is called Tie Coat Primer 6627. Interlux markets its Primocon for this purpose.
How long you wait to strip down the old paint depends on how bad the adhesion issue is and your tolerance for a rough bottom; work boats go years without stripping. Eventually though, adhesion will suffer. The rough surface will also mean more drag in the water and slower boat speed. If you do start fresh, regular application of an ablative paint-one to two coats for a single season, no more than three for long-term cruising—can prevent paint build up. For help in selecting an ablative bottom paint that is best suited for where you sail, check out our March 2013 report on favorite bottom paints by region.