diff --git a/src/p_spec.cpp b/src/p_spec.cpp
index 0ba2f01d8..bdd6820f4 100644
--- a/src/p_spec.cpp
+++ b/src/p_spec.cpp
@@ -888,6 +888,72 @@ static void SetupCeilingPortal (AStackPoint *point)
 	}
 }
 
+static bool SpreadCeilingPortal(AStackPoint *pt, fixed_t alpha, sector_t *sector)
+{
+	bool fail = false;
+	sector->validcount = validcount;
+	for(int i=0; i<sector->linecount; i++)
+	{
+		line_t *line = sector->lines[i];
+		sector_t *backsector = sector == line->frontsector? line->backsector : line->frontsector;
+		if (line->backsector == line->frontsector) continue;
+		if (backsector == NULL) { fail = true; continue; }
+		if (backsector->validcount == validcount) continue;
+		if (backsector->CeilingSkyBox == pt) continue;
+
+		// Check if the backside would map to the same visplane
+		if (backsector->CeilingSkyBox != NULL) { fail = true; continue; }
+		if (backsector->ceilingplane != sector->ceilingplane) { fail = true; continue; }
+		if (backsector->lightlevel != sector->lightlevel) { fail = true; continue; }
+		if (backsector->GetTexture(sector_t::ceiling)		!= sector->GetTexture(sector_t::ceiling)) { fail = true; continue; }
+		if (backsector->GetXOffset(sector_t::ceiling)		!= sector->GetXOffset(sector_t::ceiling)) { fail = true; continue; }
+		if (backsector->GetYOffset(sector_t::ceiling)		!= sector->GetYOffset(sector_t::ceiling)) { fail = true; continue; }
+		if (backsector->GetXScale(sector_t::ceiling)		!= sector->GetXScale(sector_t::ceiling)) { fail = true; continue; }
+		if (backsector->GetYScale(sector_t::ceiling)		!= sector->GetYScale(sector_t::ceiling)) { fail = true; continue; }
+		if (backsector->GetAngle(sector_t::ceiling)		!= sector->GetAngle(sector_t::ceiling)) { fail = true; continue; }
+		if (SpreadCeilingPortal(pt, alpha, backsector)) { fail = true; continue; }
+	}
+	if (!fail) 
+	{
+		sector->CeilingSkyBox = pt;
+		sector->SetAlpha(sector_t::ceiling, alpha);
+	}
+	return fail;
+}
+
+static bool SpreadFloorPortal(AStackPoint *pt, fixed_t alpha, sector_t *sector)
+{
+	bool fail = false;
+	sector->validcount = validcount;
+	for(int i=0; i<sector->linecount; i++)
+	{
+		line_t *line = sector->lines[i];
+		sector_t *backsector = sector == line->frontsector? line->backsector : line->frontsector;
+		if (line->backsector == line->frontsector) continue;
+		if (backsector == NULL) { fail = true; continue; }
+		if (backsector->validcount == validcount) continue;
+		if (backsector->FloorSkyBox == pt) continue;
+
+		// Check if the backside would map to the same visplane
+		if (backsector->FloorSkyBox != NULL) { fail = true; continue; }
+		if (backsector->floorplane != sector->ceilingplane) { fail = true; continue; }
+		if (backsector->lightlevel != sector->lightlevel) { fail = true; continue; }
+		if (backsector->GetTexture(sector_t::floor)		!= sector->GetTexture(sector_t::floor)) { fail = true; continue; }
+		if (backsector->GetXOffset(sector_t::floor)		!= sector->GetXOffset(sector_t::floor)) { fail = true; continue; }
+		if (backsector->GetYOffset(sector_t::floor)		!= sector->GetYOffset(sector_t::floor)) { fail = true; continue; }
+		if (backsector->GetXScale(sector_t::floor)		!= sector->GetXScale(sector_t::floor)) { fail = true; continue; }
+		if (backsector->GetYScale(sector_t::floor)		!= sector->GetYScale(sector_t::floor)) { fail = true; continue; }
+		if (backsector->GetAngle(sector_t::floor)		!= sector->GetAngle(sector_t::floor)) { fail = true; continue; }
+		if (SpreadFloorPortal(pt, alpha, backsector)) { fail = true; continue; }
+	}
+	if (!fail) 
+	{
+		sector->FloorSkyBox = pt;
+		sector->SetAlpha(sector_t::floor, alpha);
+	}
+	return fail;
+}
+
 void P_SetupPortals()
 {
 	TThinkerIterator<AStackPoint> it;
@@ -936,6 +1002,20 @@ void P_SetupPortals()
 			}
 		}
 	}
+	validcount++;
+	// Some fudging to preserve an unintended 'portal bleeding' effect caused by incomplete checks in the rendering code.
+	// Due to the addition of linedef-based portals this effect no longer works.
+	for(int i=0;i<numsectors; i++)
+	{
+		if (sectors[i].CeilingSkyBox != NULL && sectors[i].CeilingSkyBox->bAlways && sectors[i].validcount != validcount)
+		{
+			SpreadCeilingPortal(barrier_cast<AStackPoint*>(sectors[i].CeilingSkyBox), sectors[i].GetAlpha(sector_t::ceiling), &sectors[i]);
+		}
+		if (sectors[i].FloorSkyBox != NULL && sectors[i].FloorSkyBox->bAlways && sectors[i].validcount != validcount)
+		{
+			SpreadFloorPortal(barrier_cast<AStackPoint*>(sectors[i].FloorSkyBox), sectors[i].GetAlpha(sector_t::floor), &sectors[i]);
+		}
+	}
 }
 
 inline void SetPortal(sector_t *sector, int plane, AStackPoint *portal, fixed_t alpha)