Pobble
I am practising painting before I attempt to make the Mercedes Lorry Truck out of MDF.
Getting a smooth paint finish on the faces of the MDF is not a problem, but I am having trouble with the cut edges.
Is it a matter of a couple of coats of primer and plenty of sanding, or is there another way? 
I keep ending up with small hollows that I do not seem to be able to get rid of and it spoils the finish. This is just for one of my grandsons, but I still want to make it to the best of my ability.
Thanks
Brian
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Ken Martin
Hi Pobble

I have had all the same issues you are talking about with MDF.
First when you are cutting on a Scroll Saw try not to move in and out on the line, that will make the small hollows you are talking about.  If you do get the small hollows they are hard to sand smooth because the paper or sanding sticks skip over the hollow area.  
Answer:  Try using nail spongees they use for buffing toes.  These are great because they are soft little square spongees and you can compress them into the hollows.  This will finish your surface much better.

Painting the edged:  Forget the primer if you are using brush on paints.  I like to use cheap old craft paints, but you can use anything.   When I use the regular color craft paint or the Metallic craft paints, use the first coat like a primer.  Once on and dry, sand the heck out of it, that will take all the burs away.  Then re-coat with color.  You will often find the second coat of the craft paint will often be enough.  If not sand very lightly with your lightest nail sponge and put on another coat.  Now if I have the look I want I like to use a synthetic pad that equals 0000 grade steel wood (you can use 0000 grade steel wool as well), and buff the surface, before I apply a protective clear coat.  I use Krylon's Triple Clear Coat for the finial protection coat. Follow the label exactly and you will end with a finish that looks like glass.

Reason not to use primer on the wood when brush painting is the primer is usually white and the color has more trouble covering the white primer.  You will find this to be true on almost all wood surfaces when brush painting.  It is different for spray painting.  Spray painting works better with a primer, because the finish spray color goes on in very light coats.  Treat the primer when spray painting just as you did above, with heavy, than lighter sanding.

I think the most important toys I make are for the Grandson, and I too want them to be the best I can do.  If you have any questions, contact us anytime.

The photo shows some cars made of MDF.  The truck and trailer were spray painted.
DSC07047_mini.JPG  DSC07049_mini.JPG 
DSC07050_mini.JPG  DSC07051_mini.JPG 
Hope this is of some help.
Any questions feel free to ask any time.
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Pobble
Thanks Ken
   What a great variation of the Play Pals car set. I love the wheel arches. Did you cut them by hand or use a router and template?
I was planning on spray painting the Mercedes truck and was going to use 1/2" MDF
The problem I am having is highlighted in the picture below.
   As I said, the faces are not a problem, it is the cut edges and they are pitted.
This was cut on a band saw as it was just a piece of scrap to practise on.
   My daughter bought me some nail files and buffing pads in a local £1 shop so I will give them a try, but I presume they are as a finishing sanding. I need to get rid of the pitting first.
I'll have a look for some synthetic pads and I guess you are using them just before your final coat.
Thanks
Pobble
 MDF-Edge.jpg
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Ken Martin
Pobble
   Thanks, I am glad you like the little cars.  They are from the easy basics Car Carrier from Toymakingplans.com.  
I always cut out templates before I start a project, and I use a scroll saw for most of my cutting, because I am not very good with a router.
   I use the nail file sanding sticks on everything.  They start at about 100 grit and go all the way up to 600.
You can use them for anything.
   Sand the edges as best and as close as you can.  Does not have to be perfect. When you spray with the primer you will be filling some of the little holes.  Then give it a heavy sanding, and then re-prime with a heaver coat.  Now sand it down to smooth, and you will see most of the pits have gone.  If you are not satisfied with the look, prime and light sand again.
Then spray on a very light coat of color, and look for any defects you don't want in the finished toy.  Sand and spray again.  This will get you the really smooth look and feel you are looking for.
You will be amazed at the look you can get this way using simple spray paint.
Main thing is Have Fun.
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Udie
Pobble - I think Ken pretty much summed up what was the root cause of your small hollows as well as the sanding is the way to clean them up, as long as they are not deep.
   I'm going to talk about what you are calling pitting. Take a look at the link to Primer & Sealers post. Lots of good info there for you.
   We all have experienced this edge surface, because, even thou MDF does not have any grain, when you cut it, the exposed edge acts like a grained surface. The post and other articles in the Painting Tips & Techniques has many posts discussing this. Some great suggestions posted were using body filler, dries rock hard and does take a considerable amount of effort to sand smooth. MDF edges require multiple grits of sandpaper sanding, start with 120-150 and finish off with 220+. Preparation is the key. I find sealing the wood after sanding is the number one thing to do prior to painting. Painting requires multiple coats. Some members have been successful using an all purpose sealer, Spackling such as DryDex, artists Gesso which is used to coat canvas prior to painting and of course there is always the wood glue and water mixture to seal this edges which fills in the pitting as does common wood putty.
Below is a photo of what products I use frequently and a photo of the results.
   
Samples 1 A.jpg 
Gesso is a wonderful product and is available in its standard mixture, thick/heavy and clear. Very easy to apply as well as sand.
All these solutions are really great for projects that will be painted.
Hope some of this is helpful.
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Pobble
Thanks for the advice Ken and Udie.
I guess I was expecting too much from one coat of primer.
Having done some more sanding and priming I have virtually eliminated the pitting and am getting a smooth finish on the cut MDF edges, so I can now go ahead and put final coloured coats on.
This is just a practise piece to make sure I got it right before buidling the Mercedes Lorry.
Thanks again.
Pobble
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Forester
I only use MDF and I use a sealer.[smile]
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john lewman
Excellent post Udie and some priceless info for all of us. Thanks for sharing.
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john lewman
Ken,

Your work gets more and more refined and is now museum quality. I am impressed with your attention to detail.
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phantom scroller
I spray lacquer my edges then a couple of coats of paint. I don't know how long it would last outside though. 
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gonetopilbara
Hi Udie,

I am about to use Gesso and see how that goes on the mdf edges of a toy.

The photo with the Spackle seems very thin which is fine and I guess this does not need much sanding which is good. I have been using a product called PolyFilla https://www.bunnings.com.au/our-range/brands/p/poly but it seems to be thicker and is a real time consuming process to apply and sand back. Do you think it is the same product and do you have these same issues?

Bill


Udie wrote:
Pobble - I think Ken pretty much summed up what was the root cause of your small hollows as well as the sanding is the way to clean them up, as long as they are not deep.
   I'm going to talk about what you are calling pitting. Take a look at the link to Primer & Sealers post. Lots of good info there for you.
   We all have experienced this edge surface, because, even thou MDF does not have any grain, when you cut it, the exposed edge acts like a grained surface. The post and other articles in the Painting Tips & Techniques has many posts discussing this. Some great suggestions posted were using body filler, dries rock hard and does take a considerable amount of effort to sand smooth. MDF edges require multiple grits of sandpaper sanding, start with 120-150 and finish off with 220+. Preparation is the key. I find sealing the wood after sanding is the number one thing to do prior to painting. Painting requires multiple coats. Some members have been successful using an all purpose sealer, Spackling such as DryDex, artists Gesso which is used to coat canvas prior to painting and of course there is always the wood glue and water mixture to seal this edges which fills in the pitting as does common wood putty.
Below is a photo of what products I use frequently and a photo of the results.
   
Samples 1 A.jpg 
Gesso is a wonderful product and is available in its standard mixture, thick/heavy and clear. Very easy to apply as well as sand.
All these solutions are really great for projects that will be painted.
Hope some of this is helpful.
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dmjones
Wow Ken has great ideas. Anyway I use a lot of MDF and had the same problem. Someone told me to use a clear coat first. I use Zinsser clearcoat and it works extremely well. I then use spray paint to paint them. The biggest issue is drying. I have literally had some trucks that took 2 weeks to dry. Maybe I'm using the wrong paint. 
Here are 2 I di with Zinsser and spray paint on MDF. Car 1.jpg  car 2.jpg
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BadBob
dmjones wrote:
The biggest issue is drying. I have literally had some trucks that took 2 weeks to dry. Maybe I'm using the wrong paint. 


I've had so many issues with this sort of thing being caused by humidity. You should check the manufacturer's web site or the instructions on the paint container for the proper temperature and humidity range for the finish you are using.  I have found their specs to be a bit on the optimistic side.

You should also understand the difference between dry and cured for the finish you are using. As a rule of thumb dry means you can touch it and it won't stick to your fingers. Cured means its ready for whatever you want to do with it next. There finishes that can be recoated as soon as they are dry to the touch and some that you need to wait until they are fully cured to sand and refinish. Some require sanding between coats and some do not. Knowing your finishes requirements can save you a lot of grief.


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nealc
I found one coat of primer then a liberal coating of spackle, sand and prime again will be as smooth as the uncut surface .

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john lewman
nealc...Been reviewing your recent posts toda You are a skilled amd obviously experienced woodworker. Your work is impressively executed All I can aay is WOW!
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