You would have to be meticulous about placement, or you would be getting dry spots and salt buildup.  I am taking a different approach--ever seen the underbody washes they use on minesites, to clean under vehicles  before they go to the workshop? Process water at pretty good pressure and volume, turn it on, drive back and forward a couple of times. Can't fully replicate this at home really,but don't need to --this week, i am going to set up three of those big stainless Raindrop sprinklers across the drive between the trailer tracks, and  45 degree ones either side, and hook it up to the bore. Our bore water is good, just slightly more calcium than town water, so it's fine for washdown. I'd still rather not wash topsides with it, so I'm also rigging sprays facing down in the big carport, plug into the garden hose,  to wash the topsides down at the same time.  Then all I'll need to give it is a quick wash around the sides and on the deck  with the hose after its parked. Should be a big help wihen we are pulling pots--I spend about 10 minutes washing it down, at least, every day , and I won't have to get down on my knees to wash underneath.

Doing it in the next few days, I'll put up a vid when i'm done ( if it works )

Set it up on a previous trailer.

Didnt cable tie it but ran it through the tubes as it is the salt water sitting in there that causes the problems

Just used black polyu tube with the screw in micro sprinklers

Also used the small extension dripper tubes to spray into the cross members

Try just washing it down(trailer) straight way after it goes in the salt water when you launch & retrieve your boat.

People forget that your boat sits in the backing sun after you launch your boat and all that salt drys on the Trailer.

Good in theory scubafish but not always a fresh water tap where we launch our boat

but yes that would certainly help  

I have seen it setup, but I haven't spoken to anyone who said it's categorically extended the trailer life.

The issue is this...

In order for it to make any real difference, you would need to run it for quite a few minutes after launching your boat. Who has the inclination or time for that, even if you have facilities to hook it into??? And, if you have time and facilities, just use your normal washdown gear and give it a good wash.

In my view the biggest single trailer life extender is aluminium and stainless steel!!! Yes, there are steel parts, but at least on my trailer the are easy to replace.

 tectyl is a product  u can spray on a trailer or anything metal

there are different grades of it  

the viscosity changes with the number

 many years ago.. when the dead sea was only sick 

we surf launched boats every day -no chance of  rinse off - limited fresh water - sprayed tectyl once a month -preferably fortnightly, those trailers are still going - they would be on 30 years or  slightly older

couple years ago i was towing drill gear around on a barge 

the rods and everything else was pitting - they started spraying tectyl -and saved some $$

one of the range is sprayed once onto farming equipment sitting exposed to salt water on  bulk carriers 

they get to the farmer looking like new

I do not work for valvoline!

 Just finished giving my trailer a birthday .

Ive done it to two trailers now with the retick pipe, i just got some black 12mm retick pipe run it inside the box tubes both sides, put a stopper in the pipe on the rear end of the trailer, push a hose connector adaptor that will push straight into the retick pipe on the tow ball end and then clip straight onto your hose, i punched holes all the way up the tube instead of sprinklers i thought that the sprinkler heads will just pull off as your pushing the hose down the box tube with all the wires and corner bends thats in the tube. Its easy and cheap get it all from bunnings. i can clip the hose on walk away do something else while its getting fresh water all around the inside of the box tube. Does it give me longer life out of the trailer mmm well i figger that its got to be better than just putting the hose on the tow ball end and just letting the water run down the to the other end and only getting the bottom part of the box tube.

This time i also have sprayed valvoline tectyl over my brakes and springs to try it out as the reviews looked good. You spray it on black gen its in a spray can. I liked it but its a brown thick film thats left over your trailer which im ok with over the springs and on the inside of the wheels but over the rest of my trailer i wasn't so keen, and it would have been an expensive option to do the whole trailer at about $27 a can.

So i ended using a product called Lanotec its a heavy duty liquid lanolin that you can bye in 5litre container for about $70. I got my garden sprayer rigged a long hose on it put a spray nozzle on the end pushed all the way down the box tube and sprayed a heap into tube as i was pulling it back towards me. put a bucket on the other end to capture the excess.

Blocked all the wholes on the cross members and filled the cross member tubes up to the top again with the garden sprayer, pulled the bungs out captured it all, then dropped the boat in the water painted the rest of the trailer with a brush.

It leaves a clear film over the trailer but its tacky so if your going over gravel or dirt roads it would stick to it. I think i would have to do this process every few years with it as it would wear off but i think it is cheaper than replacing my trailer and hopefully i should get more life out if it. 

I still wash it down with fresh water after all that.

 No need for sprinkler systems if you buy an Ibeam constructed trailer. Would never buy a RHS trailer again.

 Still going ahead with the idea I mentioned above, just to prolong life and give the brakes and springs a good wash every time .