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HDU advice please


New Member
OK, after months of research and fiddling about I've finally found a UK supplier of HDU and the high solid content primer I believe is needed to seal the pores. I have the CNC and I have the 3D software sitting here ready to rock- I had been meaning to do a load of test pieces and the like but just haven't got round to it.

I now have a proper paying job for a pretty small HDU sign so I've got no choice but to get on with mastering HDU- the customer was looking at some US sites and loved the look of HDU- something I'm sure lots of other people will be interested in over here.

What I'm trying to find out is how to properly paint the HDU once I've chopped it into the blank. I had in mind to lightly sand the bare HDU and apply two coats of high build primer with a fine sand in between. For ease of final decoration I was thinking of adding cut out HDU letters instead of hogging them out of the main panel- I'm struggling to find a recommended adhesive I can easily source in the UK- does anyone know what type of glue the stuff the HDU companies in the US sell is?

I can only get HDU in 1500 x750mm slabs in 40mm max thickness. Are there any nifty techniques for joining slabs and hiding the mark?

The sign is 4ft wide by around 6ft and it's fitting flat to a special wall the customer is building (talk about adding pressure- can't afford to muck this job up can I:). My plan was to back the HDU with a 3mm Dibond panel cut to fit the outline of the panel- maybe making a pan out of it so I can mount the whole thing without big coach bolts and washers through the face of the HDU.

In terms of paint for the final finish I was thinking of specialised sign paint rather than DIY store stuff. Would one shot be a good choice? Alternatively I can get the full Spraylat range of sign paints but I'm not sure of the precise range to specify. Generally I'm not a huge fan of water based paint so a solvent would be my choice- if you think this is wrong please do say.

I am also thinking of adding some gold leafing to elements of this sign- would the standard sizing work? would I need to laquer/ varnish the leaf after applying?

Sorry for all the questions. If any of you HDU guys can give some tips I'd be very grateful.



New Member
Andy, check out this website www.precisionboard.com Go to the signs section and then products, they have a bonding adhesive, texture, hard coat, resin coat plus tutorials. I also have a CNC machine and have found that a good primer coat is a must. Think you're smart in giving it two coats of primer and sanding in between coats. One Shot is is a good choice also. I use the primer/filler made by either Sign Foam (Sign Prime) but will be trying Precision Boards products soon. Hope this helps, good luck!:thumb:


New Member
I only have a few HDU signs under my belt, but I usually use Jay Cooke's high build primer. I've also followed the experienced advice of others & skipped the primer altogether sometimes, like in a situation where the paint was just about the same color as the raw HDU, & the sign was up high & the texture was not a concern. I have used one shot as well as water base at one time or another.

I've used the PB240 adhesive for joining 2 pieces of raw HDU together (they say it bonds best if the HDU is slightly wet) & to make a sign longer then 8 foot, & then sandblast it with a grainframe, I did have the seam show up a little when the glue resisted the blasting more then the HDU, but I was able to minimize that with a little grinding with a dremel tool.

For installations, I've read of various techniques of routing out a channel for some aluminum supports, either flat bar, or even angle aluminum. I did a quick sketch last week to supplement a discussion, so I'll post that. It was based on a suggestion from a seasoned veteran who has done this many times, & also included backing with dibond as you mentioned, but the angle aluminum was for extra reinforcement in cases for example where the sign is not fully backed with a wall, like between 2 posts for example.


dennis j

New Member
Andy, if we are gold leafing letters we cut them out separate do the gold leaf and then attach them to the sign using silicone adhesive chalk.

We primer with two coats and then use water base paints we have had good luck with water base paints so far. Covering the gold leaf with a clear is not recomended unless the gold leaf will be where people can touch it.

For but joining sheets to gather a poylurethane glue such as Gorilla Glue works well.

Doug has covered the mounting, we also use Panel Clips for mounting. If it is double sided larger sign I will build a steel or aluminum tube frame route matching slots on the back side of both signs and sandwich the frame between the two sign faces. This frame will allow the sign to be mounted between two posts or hung from top.

Hope this helps Good Luck :smile:


New Member
Ok this is what we do,
First, sand the bare HDU (more than just lightly) you will be able to see how much smoother it gets ounce you do that. Then prime, usally two coats sanding between each. I think the best primer is the Chromatic water base primer from 1 shot, it seems to level out and get the snoothest of any I've used. Jay Cooke's works as well. I always prime my sheets first, even if you cut out letters then at least the top is primed. It is usally easier to rout letters in than cut them out, keep in mind it takes extra material and you have to sand, prime and paint each letter indivdually, but it looks nice when you are done. You could rout your backround down and leave the letters raised this will appear as cut out letters without doing it. After you are done with the primer sand it lightly before you paint. We use Ronan paints most of the time, somtimes One Shot and somtimes paint from the local hardware. Ronan works the best, dries the fastest.

As far as glue, if you are just fastening letters, regular window and door silicone works great, but you should do a test piece with exactly what your using. If you are gluing panels Gorila Glue is great stuff, but only a little bit and you have to keep pressure on it.

Fastening, what eveyone has said will work fine, but if you want no visible hardware do this: put srews or lag bolts into the wall (somthing with a hex head) and leave them stick out around an inch I'm assuming 40mm is about 1.5". Then hold the sign up were you want it and push in were the bolt are, this will make an impression in the HDU. Now lay the sign down and drill a hole the same size as the head of the screw but not all the way through the sign. Fill these holes with silicone and the sign should fit tightly onto the screws.

Hope any or all of this helps!


New Member
Guys- thank you so much- this information is very much appreciated and I feel much better about tackling this project.

I'm not sure what's going on at the moment- in nearly 20 years of sign making I have never been so busy- I'm really stuggling to keep ahead of things. I'll be starting this in around three weeks as I'm chocka block till then.

What I'll do if it's ok with you guys is post as the stages progress- your comments as I go along will be most welcome.

HDU is something that I know people here in the UK would love and there's not too many people doing it so I'm very keen to get the process cracked- having you guys to turn to really sets my mind at ease. Roxy, Dennis, Doug and Kas your are stars :)

I'll let you know how I get on- I now have the final artwork so when I get 5 minutes I'll post it so you can see what I'm needing to do.

thanks again guys