You might try a light coating of spray-on adhesive, the kind used to stick tee shirts to the printing platform of a screen-printing rig.

I've applied aluminum foil to the outside of buckets and grow tubes (NST) this way. It takes only a little bit; you could spray the gutter before applying your sheet of plastic using your fingers so as not to puncture the stuff.

Seems to me you could also pre-cut the plastic sheet and hospital-corner fold the ends so they would hold water, then just use double-backed tape to secure it in several places similar to using the spray-on adhesive.

Just a suggestion.

Thanks for the reply. I have used the spray glue and it does help secure the plastic. After I get it in place I will fill it with water and that will sit it all in place. My main problem is were I put the input line and the overfill lines, both lines are in the bottom of the gutter and plastic. I have tried silicone seal, glue, rubber gromets to seal these lines but eventually they begin to leak.(nothen seems to stick long to the plastic) Seems hard to get a good long term seal on the plastic film. Water will leak under the film and react with the aluminum. Then its a reall hasel to take it all apart, clean it, and let it dry to seal it up again. With a painted surface I would only have to shut the system down for a few hours to let it dry and fix the leak.
The system I will be putting up soon is likley to total over 100 running ft (5 20 ft runs) Trying to avoid headaches down the road. I plan on growing alot of snap peas (vines) and that would be a mess to take apart to fix a leak under the plastic. Thanks again

I'm building my system this year with vinyl downspout. Have you considered doing something like that?
What I use to waterproof holes is a male and female PVC connection with round rubber washers. I have had no leakage since switching to this technique. to get the water to completely drain, you'll need to drill a hole in the side of the connection piece 'inside' your gutter.

Hi Grizz. I have thought about swithching over to a plastic gutter or Downspout, but the reason for staying with the alum is because I have so much of it. And I am cheap. I do have a 25 ft system on the south side of my house made out of 4 inch pvc drain pipe,with petroleum jelly has the seal on the conections. Works good, but only put it up last Oct. Maybe I should just take my scrap in and buy plastic. The alum gutter looks good tho,,, matches the house and does not look like a sewer pipe on the side of the house.(wifes input here) Picture needed I think. Thanks

Easy solution to the leaky connections:

I went to a local brewery called "Hydro Brew"(here's a link: http://www.hydrobrew.com/) and bought a "bulkhead fitting". In my case, it was a valve, which I wanted to install on a plastic bucket. I'm sure you could also install a coupling or some other fitting, depending on your needs.

They sold me a rubber "grommet" that fits into a 5/16" hole drilled into the side or bottom of the bucket. Next, the valve stem (which has a "barb" on one end and a "shoulder" on the other -- nearer the actual valve) fits rather tightly into the grommet. I used a little "KY" to make it slip easier.

Now, I have a tightly-fitted, professional valve on my plastic bucket. I use it to contain nutrient for a drip-irrigation system.