what exactly is osmosis and how worried should I be?

You say the boat has now been grit blasted? I assume you mean sand blasted,
So you now have a clean hull, I would not just apply antifoul direct now but do a good job and apply some osmosis prevention coatings such as International Gelshield 200 then a primer then 2 x antifoul.

That way you are doing your best to protect your hull
Went to the boatyard today and they were priming the hull. I read out your advice (pleasantly) and was pleased to here that was exactly what they are doing but with Jotun products. Thank you:)
 
Osmosis is a bit like Covid-19 - death 'with' rather than 'from'.

A lot of surveyors have made a lot of boat repairers rich on the invariably unnecessary repairs, and even more like to specialise in providing reluctant buyers a means of escape from their purchase obligations.

Another under reported problem is that an epoxy re-coat only lasts ~ 10 years, and for a 20 - 40 year old hull that is far less time than the original hull will have taken to develop a few minor blisters.

At least two surveyors on the south coast are banned from any boat I own, so if / when I come to sell the purchaser would be politely informed if they instruct either of these pair of jokers not to bother, and if they are unhappy I simply have to find another buyer. No survey is better than a bad survey - last one I had was a regurgitation from an entirely different boat, poorly edited and full of absolute nonsense and non existent issues. Fortunately the buyer in that case was scared off by the mention of high moisture levels - hardly a surprise when a 25 year old hull has spent its entire life afloat, out of the water for just 30 minutes, yet not a single blister anywhere. This means I kept my lovely boat and now have no intention of selling it anytime soon.
 
Osmosis is a bit like Covid-19 - death 'with' rather than 'from'.

A lot of surveyors have made a lot of boat repairers rich on the invariably unnecessary repairs, and even more like to specialise in providing reluctant buyers a means of escape from their purchase obligations.

Another under reported problem is that an epoxy re-coat only lasts ~ 10 years, and for a 20 - 40 year old hull that is far less time than the original hull will have taken to develop a few minor blisters.

At least two surveyors on the south coast are banned from any boat I own, so if / when I come to sell the purchaser would be politely informed if they instruct either of these pair of jokers not to bother, and if they are unhappy I simply have to find another buyer. No survey is better than a bad survey - last one I had was a regurgitation from an entirely different boat, poorly edited and full of absolute nonsense and non existent issues. Fortunately the buyer in that case was scared off by the mention of high moisture levels - hardly a surprise when a 25 year old hull has spent its entire life afloat, out of the water for just 30 minutes, yet not a single blister anywhere. This means I kept my lovely boat and now have no intention of selling it anytime soon.

Dunno if those two were two I know who shouldn't be allowed near boats .... but

#1 : Convinced a Widow to have significant unnecessary work done to a boat that was in VGC ... and he insisted that ONLY HE should monitor the work and all parts replaced be passed to him for 'safe disposal' ....... luckily she used a close friend of mine for 'the work' ... I knew the boat as I had looked her over just a few weeks prior. We together fought that b****** over his crooked ways.

#2 : Was sent to IoW to 'survey' my boat after event requiring tow back into harbour. His instructions from Insurance Broker were to find every reason to throw out claim AND to investigate the claim already paid out previous. His report was a total fabrication and all conducted without my knowledge. It then became foundation for my bypassing Broker - who washed their hands of it after I confronted them - and bombarding the Underwriter direct. I won not only claim but also stopped the Right -Off decision of Underwriter - keeping the boat after full repairs completed.

They both made same mistake .... I am a Surveyor myself ... and could easily fault their reports item by item.
 

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