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alan0259
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Shaft
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 Posted: 00:07 - 16 Feb 2011  Post subject: Re: Stripping paint from plastic parts |
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| alan0259 wrote: | About to re-paint a belly pan and was wondering what the best method of stripping the original paintwork is?
Sand + hard work?
Chemical paint stripper?
Dry Sand?
Wet Sand?
Thanks |
Unless the part you're painting has been done several times before, you don't need to strip it, just provide an evenly keyed surface for the next coat.
Use wet and dry, anywhere between 400 and 600 grade and just get an even finish.
If you're planning on doing this kind of thing a fair bit, get yourself a random orbit electric sander (I think Machine Mart do them from about 20 quid) or, if this is a one off, hand sand it wet (with a proper sanding block, don't use your bare hand) for a little less elbow grease.
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alan0259
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Shaft
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| Posted: 00:40 - 17 Feb 2011 Post subject: |
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| alan0259 wrote: | Does sanding it wet not loose the roughness of the paper?
Thought it would be easier sanding it dry... (correct me if i'm wrong)
When do you know when to stop sanding? I started it last night for 10mins and ended up with a 1" oval white patch, the rest was just a faded blue colour...
I think it has come from the manufacturers Blue. |
Wet sanding reduces the friction by trapping a thin layer of water under the paper and between the abrasive particles, so yes, it does have the effect of making the paper less rough (but it won't matter if you're using the right grade of paper) but it also makes it easier if you're hand flatting; the other bonus is it considerably reduces dust, which is good if you're working in a confined space, especially if you're in your house.
What you need to do is sand in long strokes, as long as possible on the part you're working on, with the aim of getting a uniform matte finish; if you've got a white spot, you've broken through the colour coat and either hit primer, or gone right through to the base plastic.
That might be OK though, providing you feather the edges of the patch enough. To test, use a piece of mutton cloth or wear a latex glove and run the flat of your hand over it; if you can feel the patch, then you need to feather it some more (you might not feel it if you use your bare hand).
Make sure you use some kind of rubbing block though, if you just hold the paper in your hand, you will automatically apply uneven pressure at your finger tips and you'll put tram lines in the surface, which will end up looking like ripples in the finished paint.
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Carl_steveo
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alan0259
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| Posted: 09:13 - 17 Feb 2011 Post subject: |
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| Shaft wrote: | | alan0259 wrote: | Does sanding it wet not loose the roughness of the paper?
Thought it would be easier sanding it dry... (correct me if i'm wrong)
When do you know when to stop sanding? I started it last night for 10mins and ended up with a 1" oval white patch, the rest was just a faded blue colour...
I think it has come from the manufacturers Blue. |
Wet sanding reduces the friction by trapping a thin layer of water under the paper and between the abrasive particles, so yes, it does have the effect of making the paper less rough (but it won't matter if you're using the right grade of paper) but it also makes it easier if you're hand flatting; the other bonus is it considerably reduces dust, which is good if you're working in a confined space, especially if you're in your house.
What you need to do is sand in long strokes, as long as possible on the part you're working on, with the aim of getting a uniform matte finish; if you've got a white spot, you've broken through the colour coat and either hit primer, or gone right through to the base plastic.
That might be OK though, providing you feather the edges of the patch enough. To test, use a piece of mutton cloth or wear a latex glove and run the flat of your hand over it; if you can feel the patch, then you need to feather it some more (you might not feel it if you use your bare hand).
Make sure you use some kind of rubbing block though, if you just hold the paper in your hand, you will automatically apply uneven pressure at your finger tips and you'll put tram lines in the surface, which will end up looking like ripples in the finished paint. |
Thanks mate, makes perfect sense.
Yes, I'm going to be doing most of the prep work in the house so I will go for the wet method, thanks.
I have a feeling it's just the primer (white spot) as it didn't take much effort to get to this part.
The paper I'm using is Wilkinsons Wet and Dry (I think it's 120/150 grit).
Just done some more this afternoon, absolutely pissed it through and it only took 5 mins to take much more off, looks like this now...
https://i890.photobucket.com/albums/ac107/alan0259/hornet/IMG_0218.jpg
Im assuming the white is the old primer and the black is the bare material? How far should I take it?
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Shaft
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| Posted: 15:45 - 17 Feb 2011 Post subject: |
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Mate, that's seriously coarse paper you're using, no wonder you're going through the paint so quick, that's gonna leave the surface equivalent of a ploughed field.
You need to go much finer, 6-800 wet and you'll be needing some high build primer as a base, otherwise the paint will sink like mad.
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Shaft
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| Posted: 16:00 - 17 Feb 2011 Post subject: |
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Go fine now.
All you really needed to do was key the surface for the next coat of paint, there's no need to go beyond matting the blue top coat.
If you want to see how not smooth that is, shoot a couple of coats of primer over it now and let it dry (that won't do any harm); the paint will sink into that hole like a crater.
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Shaft
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| Posted: 16:26 - 17 Feb 2011 Post subject: |
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Go over the whole surface with the finer grade and get it even.
Then, look up your local motor factor or automotive paint supplier and get some high build primer (it's usually yellow) and put on a couple of coats, which will hopefully fill up the scratches. if it doesn't, just keep flatting and applying more primer until it's all smooth.
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Ichy
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| Posted: 10:17 - 18 Feb 2011 Post subject: |
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Personally I always give the panels a good clean with soapy water before sanding to get rid of mud, dirt and grit.
When its dry I wipe it all over with thinners to get rid of any polish or silicon residue. Same again after I've sanded it.
If the paint is in relatively good condition then you can get away with just a very light rub down to provide a key for the new paint.
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alan0259
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Shaft
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| Posted: 13:37 - 18 Feb 2011 Post subject: |
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| alan0259 wrote: | Thanks Shaft,
How long would you leave between the final base coat and the clear coat?
Heard different people saying once it's fully dry, straight away, once tacky... |
To an extent, that depends on how much paint you've put on and what kind of paint you're using.
You should be aiming to build up the colour in thin coats, not just dumping it on; assuming you're using ordinary shop bought aerosols, there should be approximate drying times on the can, but I would reckon 5-10 minutes between coats and then 20 minutes or so between the last colour coat and the first clear.
You don't want each colour coat to be completely dry before you apply the next one, but you don't want it so wet that the next coat completely dissolves the one before, because that's when you risk getting runs.
A word about temperature; the colder the room and the panel is, the longer the drying process takes, so ideally the room temperature wants to be 60 ish degrees and the panel needs to have been left in the room long enough that it's not cold to the touch.
If it is a cold room, you can speed things up by using a hairdryer on a lowish setting, but keep in mind the bit about not getting each coat totally dry, so don't go mad.
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alan0259
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| Posted: 11:44 - 21 Feb 2011 Post subject: |
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Managed to get it painted, came out pretty good but the lacquer was a bit of a problem...
https://i890.photobucket.com/albums/ac107/alan0259/hornet/IMG_0227.jpg
https://i890.photobucket.com/albums/ac107/alan0259/hornet/IMG_0226.jpg
probably put about 4/5 coats of lacquer on from a 30cm distance outside then brought it inside to dry.
The lacquer has shown up really glossy in some places and more matt in other places (slightly rough when you run your finger over it), I didnt want to keep applying it just incase it cracked or something so I just left it, still looks pretty good but can't work out why it needed caking on so much!
I've had it hung up since Saturday afternoon and was thinking of using some AutoGlym polish on it before I fit it?
The section where I went crazy with the 120 hasn't shown up either, some previous stone chips have which are underneath which i'm not too bothered about. |
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Ichy
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| Posted: 13:32 - 21 Feb 2011 Post subject: |
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As you probably realised, its not a good time of year to be spraying outside, way too damp. Preheating the part can help.
The rough lacquar may be down to overspray or not being close enough to the part. It may poish out with a bit of effort but leave it for a bit longer to give all the layers a chance to harden.
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alan0259
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Shaft
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| Posted: 01:03 - 22 Feb 2011 Post subject: |
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| alan0259 wrote: | Is it ok to spray more lacquer on top of the lacquer I put on a few days ago?
Or is it just a case of living with it? |
As Marki said, you've got some dry spots, which is either to do with conditions or technique; I don't know what the weather is like where you are, but it's a bit breezy where I am, so 30 cms would be too far away, 15 to 20 would be better, because (particularly towards the ends of each pass) the slightest breeze will blow quite a bit of paint away, before it's even got to the surface.
The other likely causes are not overlapping enough on each pass and a natural tendency to swing your arm in an arc, which means you get more paint on in the middle and less at each end of every stroke.
If you watch a pro paint, they have an entirely unnatural looking wrist and arm action (  ) which ensures that the spray nozzle is always at the same distance from the piece, along the whole length of each stroke ( ).
It is possible to do a localised lacquer recoat, but I wouldn't recommend it, unless you really know what you're doing; instead, get yourself some T-Cut and gently polish the area, then go over it again with your Autoglym stuff.
Don't be tempted to guild the lily though, once you've got it somewhere near acceptable, leave it, the next cut will see you bust through the clear coat and you'll be starting from scratch!
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Things get better with age; I'm close to being magnificent........
20 RE Interceptor, 83 Z1100A3, 83 GS650 Katana
WooHoo, I'm a Man Point Millionaire! https://www.bikechatforums.com/viewtopic.php?t=234035 |
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The last post was made 14 years, 318 days ago. Instead of replying here, would creating a new thread be more useful? |
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