This rule works in most cases. It doesn't Always work for the prepper. A plastic bucket that has never been used for any purpose before you got it does not always mean it is food grade or of the quality you want for storing food.
I let people know that I shop at Dollar Tree and other discount stores that get most of their inventory from China. I do not store food in the plastic bucket I get from China. China has a bad reputation for cutting cost corners and using lead based paints on toys. After all, there are no USFDA inspectors at the factory in China or India, or any other foreign country. Many small buckets have some seasonal paint on them, like a snowman or an Easter bunny. I would use these buckets to store nails, screws, or maybe even make a bucket of first aid supplies to give as a gift. If I were to make a batch of cookies for gift giving and I used one of these buckets, I wash it and line it with aluminum foil. That whole Mattel toy recall freaked me out.
Food grade buckets, particularly 3.5 and 5 gallon buckets I use are marked with a number 2 in the recycle symbol. Quoting from the website that provides the photo at the right, "I have learned that plastic #2, #4, and #5 are considered safest for food storage. #1 is safe but not recommended for continuous washing and reuse. How do you tell what number it is? You look at the recycle number - usually on the bottom of the container (if there's no number, defiantly DON'T use it)." I add this in because it is a concise statement backed up by hundreds of web sources. Pop over to this site and read the chemical makeup of the plastic and also get some good advice on storing herbs and spices, too!
Mylar liners can be found for purchase at www.majorsurplus.com and
www.beprepared.com and other sites. HOLD the PRESSES! I found 24 bags for $42 at www.shopsimplerliving.com .
and ten for $33 ish at https://www.readymaderesources.com