Paper weight, and what actually stops fountain pen bleed-through
The usual advice is to buy heavier paper. It is not wrong exactly, but it is not why you get bleed-through either, which is why people keep buying 120 gsm cardstock and still get feathered lines.
Three different problems, routinely confused
Precision helps here because the fixes differ. Feathering is ink spreading sideways along the fibers, turning a crisp line fuzzy. Bleed-through is ink passing all the way through so it appears as ink on the reverse side of the page. Ghosting, also called show-through, is when you can see the writing from the other side without any ink having actually got there. Feathering is about surface sizing. Bleed-through is about absorbency and thickness together. Ghosting is almost purely about opacity, which is mostly thickness. Weight only directly addresses the third problem.
Why gsm is a weaker predictor than people expect
Grams per square meter measures mass per unit area, not density, not thickness, and not surface treatment. A bulky, loosely formed 120 gsm sheet can be more absorbent than a dense, well-sized 90 gsm one, and will feather worse despite weighing a third more. This is why Rhodia and Clairefontaine at 80–90 gsm outperform most 120 gsm craft cardstock for fountain pens, and it is the single most useful thing to understand here: you are not buying weight, you are buying surface treatment, and weight is a rough proxy that fails often enough to mislead.
Sizing is the property that actually matters
Sizing is, in fact, a treatment. Historically, paper makers used gelatin. Now it's usually a synthetic. It is used to slow the speed at which liquid, or ink, penetrates the fiber matrix of the paper. Well-sized paper holds ink on the surface long enough for the solvent to evaporate rather than wick outward, which is what gives a sharp line. It also means the ink sits up on the sheet, which is where fountain pen shading and sheen come from. Poorly sized paper drinks the ink instantly: the line feathers, the color goes flat, and drying time paradoxically feels shorter because the ink has gone into the fibers of the sheet rather than staying on top to dry. Manufacturers rarely publish sizing figures, so in practice you may need to go by reputation and testing.
What this means for printing your own grids and journal sheets
Standard 80 gsm copier paper is optimized for cost and for toner, not for liquid ink, and it is usually lightly sized. It is fine for pencil and for most gel and ballpoint pens. But with a fountain pen, expect some feathering and, with a wet or broad nib, expect some bleed. Moving to a 90–100 gsm laser paper is a real improvement and is widely available. If you move to roughly 120 gsm, you gain opacity and less ghosting, but you also start to run into feed problems on consumer printers. You also gain little on feathering because the sizing likely has not changed. For grids and printable journal sheets specifically, 90–100 gsm is the sensible target.
Duplex printing changes the calculation
If you print grids and journal pages double-sided, ghosting stops being cosmetic and starts to become an annoyance. A faint grid showing through from the other side of the sheet competes with the page you are writing on, and on a dot grid it is genuinely confusing. Two adjustments help more than buying heavier paper: lighten the grid color, and reduce the line or dot weight. A grid at 40% gray rather than black at 0.3 pt rather than 0.5 pt is entirely legible from the writing side and much less visible from the other. Both the color and the weight are adjustable in the generator, and both are free.
Test it in fifteen minutes, on your own kit
Ink, nib, and paper interact, and a combination that behaves for one person may fail for another: a dry fine nib with a well-behaved ink will sit happily on paper that a wet broad nib with a saturated ink will destroy. Test rather than trust. Print the same grid on whatever papers you have, write a line with each pen and ink you actually use, wait for it to dry properly, and then look at the reverse. A 5 mm dot grid makes a good test sheet, because dots show feathering more clearly than lines do. You will learn more about your own setup in a quarter of an hour than from any amount of reading, including this. Keep the sheet; it is a reference you will use again.
A note on the recommendations you will find elsewhere
Paper reviews for fountain pens are dominated by a handful of brands, Rhodia, Clairefontaine, Tomoe River, Midori, and the enthusiasm is deserved. But those are notebook papers, sold in bound pads, and most of them do not come in loose sheets that a home printer will feed reliably. Tomoe River in particular is very thin and very well-sized, which is a wonderful combination to write on, but a difficult combination to put through a laser printer. For printing your own pages, you need to shop in a different category: laser or inkjet paper at 90–100 gsm, from a brand that publishes a brightness and opacity figure. It is a less romantic purchase but a more useful one.