Wire brush on a drill works well. Treatment wise there is a product called furtan which converts rust then prime and paint.

CRC  also have a new product out that does the same

 Not sure how much time you have but internally will be near impossible to get to. I attacked mine with a wire wheel before sending it off for acid wash and being re-galvanised. 

For my 5.6m boat trailer it cost around $300 and $400 to replace all the bolts and most rollers

As above, strip it send for acid dip and Galv.

 I am currently looking to do the same. Mine is RHS and the frame externally looks good but the galv is lifting internally. I was going to try and get a wire brush up the rhs sections and then modify a roller and try get that up there. I was going to use a product called Metalfix made by phoenix paints in Bibra Lake. 

I do have the issue of the cross members being 50x50 shs which I practically have no way of getting to the inside. I would be lucky to have a 4-5mm gap where it is welded to the frame. My thoughts were to sort the outside and paint and when they become too far gone to just cut them out and replace them.

Being a tinny trailer i'm not sure if the re-galvanise option would be cost effective?

Does anyone know if the acid wash actually clears up / eats away the loose flaking metal inside the shs? 

I did a strip down on mine a few years back, the main frame I grinded and hit with a 9" wire buff wheel before it was sent for Acid deep and Galz, the lad at the Galvanizes said it took 3 dips to get the rust off...charge was just over $4 per Kilo from memory.

Sea-kem did help me with this using his account, thanks again mate!.

All the small items I dipped in a large tub with pool acid, this basically stirps anything in a very short time frame...just don't put your head over it and breath in, you wont wake up till the next day...if ya lucky. Following this I added a few coats of good quality zinc just by brush and hand, with in a few years these items are showing medium corrosion already. they will get HDG soon after a few more mods.

This was during tough times, but I wanted get the main frame done to avoid a heavy cost later, yes you may need to budget this in, but with winter coming now's a good time to sort these projects, unless you want to buy new.

Boat trailers are probably the most easy thing to work on mechanically, even with zero skills, everything is reasonably cheap to replace and available via the trailer shops...not fkn supa cheap or the likes. Start with a cheap 5" grinder and some cutting dics..:)

 I just gave the galvanising place in Henderson a ring and asked for a rough estimate. I think I made the rust sound worse than what it is so I was quoted $5.30/kg. I'm thinking the frame stripped down may weigh 150kg. 

Hey swompa what did your frame weigh?

As mine is about the same length

Cheers fellas, convinced to the galvanising as it should be a longer term solution and from what I've read no matter how you treat it/paint the rust comes back pretty quick. As Brocko said paying the cost now will save the purchase of a new trailer or sale of the boat in a few years which I'm not keen on.

Hey sstevee mine is similar to yours with the gal flaking off mainly on the inside of box section and in areas on the cross members.

 The problem with RHS is you can't guage internally how rusted it is as I found out the hard way one time up north where the rear of the trailer broke as it was rusted out internally.

Even acid dip and regalv won't really sort the problem as it will have flake rust internally that the acid won't strip. Most galvanisers even with externally flaked rusted steel will recommend blasting.