Q:

Sealing reused plastic sacks

I always keep and reuse the plastic sacks which contain 25kg of water softener salt. Normally, I leave the bags open when I use them, but today I filled about 10 bags with gravel which will probably not be used for a number of years. I'd like to seal the tops of the bags properly just like they are when I buy them, probably involving some form of melting. Any suggestions?
EDIT: I don't want to have to buy anything to achieve this so am looking for a hacky solution. You can assume I have a very good selection of workshop tools and hot-glue gun etc. For example, my first thought is that I can probably do it by clamping lengths of metal either side of the bag and using a blowtorch to heat one or both sides to melt the plastic together.

A:

In the youtube-video How to Seal Plastic Bag at Home #Ludvic #Maker, demonstrates a rather hackish way of sealing plastic bags. In essence what he does is lay the plastic bag flat, put a piece of paper over the top of the bag and then use a soldering iron to transfer heat through the paper and melt close the plastic bag.
Variation over this method could be to use other heat sources:

soldering iron (as suggested in video)
edge of clothes iron
hot glue gun (possibly without glue?) (that is use the heated tip of the glue gun, not the actual gluing capabilities :-) )
a hair iron (the ones for straightening hair)
some other heated metal-strip

The point of using the paper between the bag and the heat source, I contemplate is to avoid getting melted plastic on your heat source. 
I would also consider double sealing them for additional strength. That is don't run the sealing strip only once, but twice across the top of the bag.

