Greg, there are differing views on this. I think for an older boat (newer than mine, but still an '88), if it doesn't have any blisters by now, the hull is probably in good shape (not to say it won't ever have any blisters), so doesn't need major fix/preventive work. The concern with barrier coating an older boat is that there will likely still be moisture in the hull (there is general consensus that gelcoat is somewhat porous), even if you dry it out over the winter, there will still be some in the spring. The risk is that the barrier coat will trap this moisture in the hull, and potentially cause blisters. On the other hand, some argue that as long as it is mostly dried out (< 5% moisture) ie. if you tape a piece of plastic on the hull, leave it in the sun, and if no condensation shows up inside the plastic over a 24 hour period, it's probably o.k. to consider barrier coating. If you do coat it, ensure the right thickness (I think usually about 10 mls.) to achieve your desired protection.
There is lots written about this, so you may want to do your own reading.
Good luck and let us know what you decide.
Frank.