Plywood - So much choice

[Astralstroll]

Now I have some tools to enable me to plan a little more about building a dob, the next thing on the list is the type of ply to use. The clear beech ply looks the best, but is rather pricy. Is it worth the extra money if I'm more than likely to paint the scope after its built?

once the grade is chosen, where is the best place to buy? I'm guessing a decent lumber yard is much preferable to the likes of B&Q.

Dave.

[Steep]

On where to buy, it depends how much ply you're going to need, a diy store that's easy to get to might be a better source for a sheet or two than a sawmill/yard that takes a bit of travelling all other things being equal.

[Stargazer51N]

I've made quite a lot of things out of ply over the years and have found B&Q to be a reasonable price. Some B&Q stores have cutting machines and will cut pieces to size for you more accurately than can easily be done at home (can't remember how many cuts they officially do for free but I've found if you go at a quiete time in the week they can be very accommodating). Wherever you decide to buy from I'd recommend you use 'Marine' or WBP (waterproof bonded ply) so that the damp can't get in and cause it to 'blow out'.

[pete_l]

The clear beech ply looks the best, but is rather pricy. Is it worth the extra money if I'm more than likely to paint the scope after its built?

IMHO, no: not really. It's supposed to not contain any voids or holes on the inner laminates, but for a 'scope base that's not really important and using "the best possible" doesn't make you a better woodworker . As Stagazer51N says, just use WBP so it won't fall apart from dew, use waterproof glue (I like polyurethane, but don't let it get on your hands and clamp the joints as the glue/foam expands). Then sand it all smooth and give the edges an extra coat of paint.

[Scott]

If you've got any mates in the building game, ask them if they have an account at b&q. they get discounted prices (sometimes quite large)

[swamp thing]

Aaah a Dob. This would answer my question in your mirror cell thread

On ply, much depends on your design. Are you building a Kriege type (obsession) truss scope?

[Steve Ward]

I assume you mean BIRCH not BEECH ?

Beech faced anything is not recommended for external use in any form , will go black and dotey at the hint of moisture.

WPB and Marine ply are two completely different things , I'd recommend trawling round a boatyard ( your location suggests this won't be hard ) for a supply of genuine marine grade.

I make a lot of dashboards for open topped classic car restorations and wouldn't use anything other than the genuine thing for the substrates.

[Moonshane]

having used lots of other plys, my view is that even if painting I'll only ever use birch ply in the future. it's simply a completely different quality to anything else I have used. it's more attractive, more stable, more hardwearing, less full of holes and yes, OK more expensive.

it's about twice the price but if you buy a full sheet at £50-60 or a cheaper alternative at £25-30, does the additional cost really matter?

[Ajohn]

I have always used straight ordinary exterior grade ply even for furniture but edged with hardwood mouldings. It can be sanded, stained and varnished just like any other timber. The quality of the external faces does vary so it's best to look around.

The biggest problem with this type of ply is cutting it. Jig saws seem to be popular around here but a small circular saw is a much better option used correctly. Going too big makes these saws more difficult to handle. Personally I wouldn't buy a battery operated one either. The one I use most often has a 160 odd mm dia blade. I've had it a long long time Makita DIY range so it wont cut as deeply as most saws of this size. The blades that they come with are useless for ply. Freud do some excellent thin kerf blades. For ply a 40 tooth should be fine at this size. They are rather cleverly made blades and produce a fairly high quality finish. On hardwood for instance edges just need a light sanding.

When using the saw make sure that the blade is set square or at the angle that needs to be cut and set it so that it only goes a few mms past the depth of the material being cut. 3 or 4 mm is fine.

Sheets of ply need supporting when they are cut. Easiest way to do that is to clamp lengths of lath etc some way in from each end. Even though these finish up being part cut by the saw they can be used over and over again.

Best thing to use to cut straight is a straight edge. Siverline do some that are intended for plastering but work well. I found a 6ft one adeqaute for a long time but eventually bought an 8ft one as well which is often a bit cumbersome to use. Trend also do something that comes with clamps. Using these is just a case of measuring the position of the blade from the edge of the saws sole plate and off setting the straight edge to accommodate it. It still pays to mark out where the cut needs to go to save making mistakes.

Other straight edges can be used - the edges of ply and mdf sheets are often very straight. Say a 150mm with of that could be cut off any old way. Aluminium extrusions or even a straight piece of timber. Routers can be run along straight edges as well. Even jig saws but these often splinter ply what ever type it is. They are also handy for ripping length of timber on other types of saw. The straight edge is clamped to the work and run along the edge of the table of the saw. It's far more precise and and easier to handle than the fence on some machines.

I have often managed to cut up full sized sheets of several things on top of a single DoItAll workmate. 2 wouldn't be a bad idea really but the fact that the piece being cut off remains in place makes the use of one feasible. Some thought is some times needed to avoid the saw fouling on the various clamps holding the laths and straight edge in place.

I did own a well made table saw for a while but found the straight edge and circular saw adequate and more versatile even though the table saw had a panel cutting attachment so I sold it as it was hardly ever used. I still own a radial arm saw but to be honest it wont really do anything that a straight edge and circular saw can do.

Next step up from exterior grade ply in my book is marine ply.

John

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