I found an answer here. It is possible to do this in the Lightroom Print Module by choosing the 8 x 10 preset from the Template Browser (the 8 x 10 preset has borders on all four sides). To get a border on just the left side, I modified the Layout by adjusting the margins (see below), then saved as a User template preset for future use. A single left margin works for me because (a) it doesn't crop my image vertically and (b) I only need to make one cut in order to get a borderless print
In Lightroom- Crop your image to 4:5 ratio, (ie.same as 8:10)
Export your image as JPG.
Do Not tick resize in the export dialog. ( and entering a 'ppi' is irrelevant)
If you want wider or colored borders, logos, etc, design you print in the Print module and "Save to JPG". Here you can set size and ppi.
The basic answer to your question is "Crop 4:5"
Do you mean "When you Send out an image to be printed you get funny results"? It's not clear whether you have started printing yourself, you bought a printer, or the printing service you are sending your images to is sending back prints that are exactly the aspect ratio you sent them.
This is a common problem with print services. you have to Specify Exactly what you want printer, aspect ratio, on what size paper even if there are white boarders on the sides or top and bottom. Otherwise, if you din't specify exactly what you want, they crop the image to fill all or most all of the paper.
Even if you fill in the spaces that might be a white board from AR of image to AR of paper the print service may still Crop Out that boarder and Crop the actual image to Fit the paper size.
LRMogrify2 plug in http://www.photographers-toolbox.com/ has a feature which allows you to extend the canvas on export and thereby enable you to create files with the necessary blank borders to make aspect ratios of image and paper match. Settings permit export of a batch from LR and Mogrify's plug in features will add margins correctly for both portrait and landscape orientation - perfect for sending JPG files of a different aspect ratio to an external printer using standard aspect ratio paper. LRMogrify is a sophisticated tool and will enable you to do much else besides - I find it indispensible in many different cases such as borders, captions &c. You will have a learning curve in using it - more long winded on PC than Mac
I crop in the viewfinder.
I never had a printer. I have always printed commercially.
I said that I got "funny results" because I did not want to go into detail. The service where I print has a Web-based interface for uploading photos, selecting paper size and quantity, etc. The next paragraph explains what I did and then what happened.
In the Lightroom "Export to Hard Drive" dialog in the "Image Sizing" section, I scaled my 2:3 aspect ratio image so that the long side was exactly 10 inches (making the short side 6.67 inches). I did this by setting Long side equal to the actual pixel dimension of the long side of the image, and setting the resolution equal to the pixel dimension divided by 10. Example: for a 4000 x 6000 image, specify "Long Side" = 6000 pixels and Resolution = 600 pixels per inch. I also check "Don't Enlarge" as an extra precaution.
When I upload one of these exported jpgs, the print service gives me a "crop warning" and shows me how the image is going to be cropped by default. They completely ignore the pixel dimensions and resolution in the jpg I sent them! What they have done instead is scale the image so that the SHORT dimension is 8" (making the long dimension 12.5"), then placing an 8 x 10 crop box over it and allowing me to slide the box up and down to determine which 2.5" I want to lose. That's more sad than funny.
Thanks for the pointer to Mogrify2. But as I posted earlier in this discussion, I already found a solution. The Lightroom Print Module lets you add top, bottom, left, and right margins to the image, and then allows you to save a jpeg. That's the easiest solution, although I'm still having problems getting it to do it the way I want in some cases. I'll probably make a separate post about those problems.