When anyone is using "used" buckets, tubs, crates, or in your case barrels, in their set up, the question about whether it's safe or not isn't about the actual plastic, because the vast majority of plastics are totally fish safe, and HDPE is fine. No, the big question is, what has that plastic been in contact with in its previous life.
Plastics have what is called "absorption rates" and over time their contents can leach into the polymer. Then when you use said plastic the previous chemicals can, over time, leach back out, in our case, into our aquarium system, potentially causing very bad problems.
Generally speaking a thorough multiple washing, preferably with a high powered jet wash, and very hot water will do the trick, no cleaning chemicals for obvious reasons.
However the only time I personally won't go near a barrel, whether it's been cleaned or not, is if it's had toxic waste in, you've probably seen them, the type of waste that usually has a sign on that signals it's bad for the environment. Barrels that have had food stuffs in, or in your case cosmetic products tend not to be so bad once washed out as highlighted above.
I work in plastics recycling and I find useful things all the time which I take home. The picture below shows a few things I collected from work last year just prior to me setting up my 360g fibreglass tank. That blue barrel had contained some flavourings for the food industry. A thorough wash and I was good to go. That barrel has been in my system for just over a year now, no problems at all.
If you've got some very expensive fish and you just don't want to risk it you can buy brand new barrels like this on line anyway, and they tend to be reasonably priced. That way you have 100% peace of mind. Hope this helps.
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