How do I clean and treat my Aluminium propeller?

[Yngmar]

Boat has a fixed 3-bladed Volvo aluminium alloy propeller + cone, which after some epic struggle I've managed to get off the saildrive (this involved Plusgas, home brewed release agent, a heat gun and finally dangling my fat self from a 2m spare cradle leg for leverage). I've bought some hydrochloric acid in the guise of brick/patio cleaner, falsely assuming the prop would be bronze. Now that it turned out to be aluminium, I'm not sure if it's safe to dunk it in that, or how else to get the random bits of fouling (mostly scraped off already), old paint and old primer off (for all I care the primer can stay on, if it will stay on for good). So is the acid bath safe or will it eat the material? If not safe, how do I get the crud off?

And secondly, once it's off, what do I put on there? I have some alu etch primer that worked very well on the rudder quadrant (also alu), and then I guess I just paint some Trilux or something on top of that? I believe it's the original 15 year old propeller, and it looks to be in very good condition - I'd like to keep it that way, and also get it to not foul up too much (although that last part seems to be a bit of an unsolved problem from my research).

 

[Tranona]

You will be lucky to get it back to any decent condition. It has a rubber bush in the centre and you may well find corrosion around the edges of the bush, particularly on the front face. suggest you dip just the blades in either brick cleaner or kettle cleaner which will remove the crud. Etch primer then Trilux is reasonably effective in keeping it clean, but long term a folding or feathering 3 blade prop is a good investment.

 

[Halo]

Best thing I have done on my boat is swap the fixed ally prop for a flex o fold 3 blader. Costs a fortune but well worth it IMHO.
I have found that it is hard to get paint to stick to the etch primer (even on a saildrive housing) but have found that Hammerite Special Metals Primer works like a charm on ally and costs a small fraction of the cost of Trilux, etch primer and undercoat.
Also if you do use etch primer the part and paint need to be warm and it stinks really bad - so do it now before the cold weather means you need to do it in the house and drive SWMBO nuts like I did

 

[vyv_cox]

Hydrochloric acid reacts vigorously with some grades of aluminium but far less so with others. I would be very careful before immersing your prop in it. A weak alkali such as washing soda can be very effective, especially warm. Personally I would use wet and dry papers, not too coarse as you risk removing any anodising.

 

[Yngmar]

Thanks for the advice. I've had a go at this. I didn't dare the acid bath after all, and cleaned it up the same way I cleaned my quadrant, as that has worked well. This was with an Aggresso rotary wire brush in a drill (I love that brush - I've had it for over a decade now and it has never lost or bent one bristle). Sounds quite brutal, but didn't harm the alloy much (being a propeller, it's probably a very different alloy than the stuff used for spars). This took off the barnacle glue, old paint and most of the old black primer, the majority of which had already come off during pressure washing (no anodising present). Then Dremel with a bag of cheap rotary wire brushes to clean up the corners. Would've painted it today had my Dremel not packed up just before finishing the job (grumble).

That's the exact same procedure I've done on the quadrant, with a good scrub in dishwashing liquid and rinse before putting on the etch primer and then black Hammerite. Happy with how that turned out:

In the process I've also uncovered a stamp and thus positively identified the propeller as a Volvo 17 x 15 Part No 21381279, which does indeed seem to come with black primer from new. Did not find a rubber bush in the centre. Apart from two insignificant areas of pitting the prop is in very good condition. It also has some bitemarks near the front of each blade that perfectly match the teeth of my Stripper rope cutter. I wonder how that happened, there's no way for the Stripper to go there if fitted correctly. Maybe it once wasn't.

That just leaves the painting. The etch primer is pretty obvious, but I wonder if there should be another layer of protection before the antifouling. What does one use for that?

A feathering prop would be nice, but the refitting is costing a fortune as it is, so that will have to go on the wishlist.

 

[vyv_cox]

I have posted several photos previously showing my prop at end of season using Velox antifouling. For three years now the prop has been completely free of any fouling when hauled out. This year the boat has been in the water since end of April in Greece, including a month stationary in the marina at Lakki. The hull, conventional A/F, was not too bad, scrubbed almost free of fouling in the water. The keel, Coppercoat now about 8 years old, suffered badly in the static month and has a lot of tubeworm shell with vegetation attached to that. The prop is completely clean. The P-bracket, also done in Velox, has a little tubeworm right at the join with the hull and a frond of vegetation. Photos in about three weeks but once again Velox seems to have been highly effective.

 

[Tranona]

In the process I've also uncovered a stamp and thus positively identified the propeller as a Volvo 17 x 15 Part No 21381279, which does indeed seem to come with black primer from new. Did not find a rubber bush in the centre. Apart from two insignificant areas of pitting the prop is in very good condition. It also has some bitemarks near the front of each blade that perfectly match the teeth of my Stripper rope cutter. I wonder how that happened, there's no way for the Stripper to go there if fitted correctly. Maybe it once wasn't.

If you look carefully at the ends you will see the centre is a pressed in bush. You can't actually see the rubber unless it is starting to fail as the ends are metal. The splines are stainless and the rubber serves two purposes, first to act as a cushion and second to isolate the prop from the shaft. The nicks out of the prop were probably caused by picking up some debris which the cutter chopped. The edges of the blades are very easily damaged.

 

[scottie]

Grease the propshaft splines with blue VP grease same as stern gland you don't have and remove prop annually
The locking bolt is meant to be replaced every other year or use loctite at least

 

[Yngmar]

Amazon gets credit for sending me a new Dremel with next day delivery (and didn't balk about the 2 year manufacturer warranty). I've used it to finish the detail cleanup, gave the prop a nice hand sanding for good measure, a wash and then brushed on the acid etch primer. Good stuff by the way, Xylene based, got it from http://militaryvehiclepaint.co.uk/ (also as Fascincating Finishes Ltd on Amazon). Easy to apply and sticks like ice cream to a kid. Must remember to buy the matching special thinner or your brush is a write-off though (Harris no-loss synthetic brush, £1.25 at Asda. In case you forget the special thinner, ahem.)

Before and after pictures:

You can see the teeth marks from the rope cutter near the top blade. I've taken a multimeter to it and the splines are not isolated from the propeller, although that doesn't mean there is no rubber bush, it may just be worn, as the center looks indeed to be pressed in.

I'm now shopping around for a two part epoxy coat to go between my grey primer and antifoul (Velox sounds good, although I'm half tempted to give Coppercoat a try - in theory it should stick on top of epoxy).

The prop has to come off whenever the anode needs replacing, so that's how often that will happen in the future (unless there's other mishaps). It probably hasn't been off in years and was an utter ☠!✻★#☁₪⌁!★☆ to undo the cone (lots of calcium deposits between nylon washer and cone). The locking bolt was tight but came undone without much ado. Will Loctite that in place as well as the bolts holding the Stripper.

 

[vyv_cox]

It is not normally possible to apply an epoxy on top of a conventional paint, the adhesion will fail. This would apply to Coppercoat and and epoxy primer.

 

[Yngmar]

It is not normally possible to apply an epoxy on top of a conventional paint, the adhesion will fail. This would apply to Coppercoat and and epoxy primer.

Hmm, that makes the choice easy then. Thanks

 

[Tranona]

I discussed Coppercoating the propeller with the manufacturer. Definite no-no on the basis that it won't stick. Used Trilux because I have had god experience with it on both aluminium and bronze propellers.

 

[Slowtack]

On my saildrive I replaced my VP three blade folder with a fixed aluminium two blade. Propulsion was fine but on lifting out i found the paint had striped to bare metal on the blades from tip to root. I have been recommended to use Hammerite. I expect I will need to use an acid etch primer. Any advice or opinions welcome as regards suitable long lasting paint.

 

[vyv_cox]

I find that Hammerite special metals primer, the old solvent based rather than the newer water based, which I have not tried, adheres superbly to manganese bronze and to whatever the blades of my Autoprop are made from. I have also used the same stuff on the galvanised roller shutter door of my workshop, where it also lasted well. That would be my choice on aluminium also.

 

[Slowtack]

Thanks Vyv - sounds good will try to source that.